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What begins as 1 cell has the potential to grow into 100 trillion cells. The beginning of this process is called meiosis. 

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The urge to procreate is a fundamental part of all life. In humans, when 2 individuals each create one cell of 23 chromosomes, and those cells are brought together through fertilization, the possibility of a new life begins.

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Sperm is found in the testicles. There is over 1/2 mile of tiny tubes that house the sperm.

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For men, these cells are found in the sperm. Males produce 1000 new sperm every second and over 2 trillion over a lifetime.

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The egg is encased in a rich protein shell called the zona. For fertilization to happen, the sperm must break through the shell.

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A female has all the eggs she will ever have for reproduction while she is still in the womb. These are released each month by the ovaries as part of a woman's menstrual cycle.

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On any given day, only a few sperm will make it to the egg.

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When male sperm are released into a female, they are led to the egg inside the uterus. Once they come together, they have one mission...to create a viable embryo.

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50 percent of all fertilized eggs fail to become embryos.

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Sperm releases its chromosomes and the egg chromosomes line up next to it- and then the cell divides.

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While the fertilized egg is moving through the fallopian tubes, the cells continue to divide, gradually creating the building blocks needed to construct an embryo.

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Striped of protection, the cells can be attacked by the mother's white blood cells. For protection, the cells release chemicals that suppress her immune system.

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The cells break out of the zona and land on the blood-rich lining of the mother's uterus. This process is called basalis.

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The basalis is about the size of a poppy seed. It is here, deep inside the uterine lining, that the cells start to organize themselves into an embryo. This process is called "gastrulation".

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The embryo at this stage is about 1/10 of an inch long.

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Scientists believe at the beginning of gastrulation, some cells begin moving toward the center, then dive down creating a new lower layer. Eventually, there will be 3 layers of cells, all having different functions. The bottom layer will become the lungs, liver, and digestive system, the middle layer will become the heart, muscles, bones, and blood, and the top layer will become the spinal cord, brain, nervous system, skin, and hair.

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Between 3 and 4 1/2 weeks, the neural tube is in place and the brain has started to form. Blood vessels are forming and the heart starts to beat. The beginning of arms and legs also start to form.

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When can I take a pregnancy test?

By the end of this week you may be able to get a positive pregnancy test. Pregnancy tests work by detecting the presence of a hormone called human chorionic gonadotropin (hCG) in your urine. Most home pregnancy tests claim to be accurate on the day you miss your period, but even then, the amount of hCG in urine at this time can vary a great deal from one woman to another. If you test too soon, you may get a false negative pregnancy test or an unclear result like a faint line. If this happens, test again in a few days.

Implantation happens this week

Your developing baby has travelled down the fallopian tube and is starting to implant itself in the lining of your uterus. At this time, 15 to 25 percent of women experience implantation bleeding, which is a light bleeding that happens about six to 12 days after conception. You may feel implantation cramps as well.

Early pregnancy symptoms

Most women don't feel anything until they've missed a period, but you may notice bloating, cramping, or spotting this week. Your breasts may also be more tender than usual and you may have a heightened sense of smell, one of the earliest pregnancy symptoms. So if your partner, your house, or your dog suddenly smells different to you, thank your surging hormones.

3 weeks is how many months?

You're in your first month!

Baby development at 3 weeks

Your growing baby

Your developing baby is a tiny ball (called a blastocyst) of several hundred cells that are multiplying and burrowing into the lining of your uterus. The cells in the middle will become the embryo. The cells on the outside will become the placenta, the pancake-shaped organ that delivers oxygen and nutrients to your baby and carries away waste.

Connecting to you

Your little blastocyst is receiving oxygen and nutrients (and discarding waste) through a primitive circulation system made up of microscopic tunnels that connect your developing baby to the blood vessels in your uterine wall. The placenta will eventually take over this task around the end of the first trimester.

Having twins?

It's too soon to know. But it's still fun to see what fraternal twins look like in the womb this week. You can also learn about your likelihood of having twins or more.

3 weeks pregnant body

Early pregnancy hormones

The cells that will become the placenta are pumping out the pregnancy hormone hCG. It tells your ovaries to stop releasing eggs and keep producing progesterone, which prevents your uterus from shedding its lining - and its tiny passenger. Once there's enough hCG in your urine, you'll get a positive pregnancy test result.

Fluids are filling your uterus

Amniotic fluid is beginning to collect within the amniotic sac. This fluid will cushion your baby in the weeks and months ahead, and it's what may come gushing out of you if your water breaks before or during labor.

Pregnancy symptoms during week 3

No pregnancy symptoms?

Some women feel pregnant even before the test is positive, but most don't notice anything yet. If you have symptoms this week, some of them may feel like PMS. Don't worry if you don't feel anything yet. Even at 5 weeks pregnant, only half of women feel pregnancy symptoms.

Gas and bloating

The hormone progesterone relaxes muscles throughout your body, including in your digestive tract. These relaxed muscles slow down digestion, which can lead to gas and bloating and create uncomfortable sensations in your gut. About half of pregnant women experience constipation at some point during their pregnancy. To keep things moving, stay hydrated and eat high-fiber foods like whole grains, fruits, and vegetables.

Sore breasts

Many women say that the tenderness they feel is an exaggerated version of how their breasts feel before a period. Some women also notice their nipples getting darker during pregnancy.

Pregnancy checklist at 3 weeks pregnant

Pay attention to your emotions

When you're waiting to learn whether you're pregnant or not, or just finding out, it's normal to be more anxious than usual. If you're feeling stressed or worried, talk to your partner or a trusted friend. Or, try writing down everything that's bothering you. Journaling can improve your emotional well-being, mental clarity, and even physical health.

Avoid overheating

Hot baths are okay during pregnancy as long as they aren't too hot. But avoid steam baths, hot tubs, and saunas. Elevated body temperature, especially early in pregnancy, has been associated with an increased risk of neural tube defects in babies.

Eat nutritious meals and snacks

Eat foods that support conception and pregnancy such as fruits and vegetables, low-mercury fish, and whole grains. Choose foods containing vitamin C (like strawberries, citrus fruits, bell peppers, and tomatoes), iron (like beef, poultry, soy products, and spinach), and calcium (like Greek yogurt, fortified cereal, and pasteurized cheese). For snack ideas, check out 10 of our favorite healthy snacks for expecting moms.

Cut down on coffee

While you're trying to conceive and once you get pregnant, experts recommend limiting your intake of caffeine to about one cup of coffee a day. It's important to watch your consumption of coffee and caffeine overall because excessive caffeine can affect your pregnancy and your baby. See how much caffeine is in different foods and drinks.

Get help quitting

If you need help quitting smoking, drinking, or taking drugs, talk to your healthcare provider and ask for a referral to a program or counselor.

Improve your sleep

Rest while you can so that when pregnancy-related sleep disturbances hit in a few months, you'll be ready for them. Form better habits around sleep and work on good sleep practices like establishing a regular bedtime routine and making your bedroom a sleep sanctuary.

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When can I take a pregnancy test? By the end of this week you may be able to get a positive pregnancy test. Pregnancy tests work by detecting the presence of a hormone called human chorionic gonadotropin (hCG) in your urine.

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The embryo is only about 1/5 of an inch long. The backbone has begun to form into a tail that will disappear in a few weeks. A large brain has started to develop and on the side of the head, an eye appears.

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At 7 weeks, all male and female embryos look the same. Genes powered by DNA will help gonads develop into ovaries or a penis. By 18 weeks, you can tell the baby's sex through an ultrasound.

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The embryo becomes a fetus. It is less than one inch long and under 1/3 ounce. All organs are in place but not all working. Villi from the placenta feed the fetus nutrients and oxygen.

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...the fetus will continue to grow to almost 400 times it's current size.

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The second trimester includes weeks 13 through 27. The baby grows larger and many women begin to show a larger belly. Women start to feel the baby move and they have an increased appetite as they continue to nourish the baby.

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A pregnancy lasts for about 40 weeks. The weeks are grouped into three trimesters. The second trimester includes weeks 13 through 27 of a pregnancy.

In the second trimester, the baby grows larger and stronger and many women begin showing a larger belly. Most women find that the second trimester is a lot easier than the first, but it's still important to be informed about your pregnancy during the second trimester. Understanding your pregnancy week by week can help you make informed decisions and prepare for the big changes ahead.

During the second trimester of pregnancy, symptoms that you may have experienced during the first trimester begin to improve. Many women report that nausea and fatigue begin to lessen and they consider the second trimester the easiest and most enjoyable part of their pregnancy.

The following changes and symptoms may occur:

the uterus expands

you begin to show a larger abdomen

dizziness or lightheadedness due to lower blood pressure

feeling the baby move

body aches

increased appetite

stretch marks on the stomach, breast, thighs, or buttocks

skin changes, like darkening of the skin around your nipples, or patches of darker skin

itching

swelling of the ankles or hands

Call your doctor if you experience any of these symptoms:

The baby's organs become fully developed during the second trimester. The baby can also start to hear and swallow. Small hairs become noticeable. Later on in the second trimester, the baby will begin to move around. It will develop sleeping and waking cycles that a pregnant woman will begin to notice.

According to the American Pregnancy Association, by the end of the second trimester the baby will be around 14 inches in length and weigh a little over two pounds.

Women should see a doctor about every two to four weeks during the second trimester of pregnancy. Tests that the doctor may perform during a visit include:

measuring your blood pressure

checking your weight

ultrasound

diabetes screening with blood tests

birth defect and other genetic screening tests

amniocentesis

During the second trimester, your doctor can use an ultrasound test to determine whether or not your baby is a boy or a girl. Deciding whether or not you want to know the sex of the baby before you give birth is your own choice.

It's important to be aware of what to do and what to avoid as your pregnancy continues. This will help you take care of yourself and your developing baby.

What to do

Continue to take prenatal vitamins.

Exercise regularly.

Work out your pelvic floor by doing Kegel exercises.

Eat a diet high in fruits, vegetables, low-fat forms of protein, and fiber.

Drink lots of water.

Eat enough calories (about 300 calories more than normal).

Keep your teeth and gums healthy. Poor dental hygiene is linked to premature labor.

What to avoid

strenuous exercise or strength training that could cause an injury to your stomach

alcohol

caffeine (no more than one cup of coffee or tea per day)

smoking

illegal drugs

raw fish or smoked seafood

shark, swordfish, mackerel, or white snapper fish (they have high levels of mercury)

raw sprouts

cat litter, which can carry a parasite that causes toxoplasmosis

unpasteurized milk or other dairy products

deli meats or hot dogs

the following prescription drugs: isotretinoin (Accutane) for acne, acitretin (Soriatane) for psoriasis, thalidomide (Thalomid), and ACE inhibitors for high blood pressure

Ask your doctor if you have any concerns about the prescription drugs or supplements you are taking.

Although there are still several weeks left in the pregnancy, you may want to plan for delivery earlier to help make the third trimester less stressful. Here are some things you can do now to prepare for birth:

Take prenatal education classes that are offered locally.

Consider classes on breastfeeding, infant CPR, first aid, and parenting.

Educate yourself with online research.

Watch birth videos on YouTube that are natural and not frightening.

Tour the hospital or birth center where you will be giving birth.

Make a nursery or space in your house or apartment for the newborn baby.

Consider whether or not you want to take medication for the pain during delivery.

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A pregnancy lasts for about 40 weeks. The weeks are grouped into three trimesters. The second trimester includes weeks 13 through 27 of a pregnancy. In the second trimester, the baby grows larger and stronger and many women begin showing a larger belly.

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hard bone starts to be seen.

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begin to hear sounds.

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The third trimester of your pregnancy starts at 28 weeks and lasts until you give birth. Most babies are born around 40 weeks.

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Third Trimester of Pregnancy: Fetal Development

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The third trimester of pregnancy starts in week 28 of your pregnancy and lasts until you give birth, which is around the 40th week. The third trimester can be tiring and more uncomfortable as your baby takes up more room and you experience: Backaches: Pregnancy hormones relax the connective tissue that holds your bones in [...]

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the fetus' job is to grow. Fat reserves line the brain, increasing nerve impulses and brain power. When the baby's need for fat is too great for the mother to provide, the baby must be born to survive and labor begins.

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There are several stages of labor as a woman's cervix dilates and effaces. Contractions become stronger over time until the cervix opens to about 4 inches to accommodate a baby's head.

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Stages of labor and birth: Baby, it's time!

Labor is a natural process. Here's what to expect during the three stages of labor and birth - and what you can do to promote comfort.
By Mayo Clinic Staff
Labor is a unique experience. Sometimes it's over in a matter of hours. In other cases, labor tests a mother's physical and emotional stamina.

You won't know how labor and childbirth will unfold until it happens. However, you can prepare by understanding the typical sequence of events.

Stage 1: Early labor and active labor

The first stage of labor and birth occurs when you begin to feel persistent contractions. These contractions become stronger, more regular and more frequent over time. They cause the cervix to open (dilate) and soften as well as shorten and thin (efface) to allow your baby to move into the birth canal.

The first stage is the longest of the three stages. It's actually divided into two phases - early labor and active labor.

Early labor

During early labor, your cervix dilates and effaces. You'll likely feel mild, irregular contractions.

As your cervix begins to open, you might notice a clear pink or slightly bloody discharge from your vagina. This is likely the mucus plug that blocks the cervical opening during pregnancy.

How long it lasts: Early labor is unpredictable. For first-time moms, the average length varies from hours to days. It's often shorter for subsequent deliveries.

What you can do: For many women, early labor isn't particularly uncomfortable, but contractions may be more intense for some. Try to stay relaxed.

To promote comfort during early labor:

Go for a walk

Take a shower or bath

Listen to relaxing music

Try breathing or relaxation techniques taught in childbirth class

Change positions

If you're having an uncomplicated pregnancy, you may spend most of your early labor at home until your contractions start to increase in frequency and intensity. Your health care provider will instruct you on when to leave for the hospital or birthing center. If your water breaks or you experience significant vaginal bleeding, call your health care provider right away.

Active labor

During active labor, your cervix will dilate from 6 centimeters (cm) to 10 cm. Your contractions will become stronger, closer together and regular. Your legs might cramp, and you might feel nauseated. You might feel your water break - if it hasn't already - and experience increasing pressure in your back. If you haven't headed to your labor and delivery facility yet, now's the time.

Don't be surprised if your initial excitement wanes as labor progresses and your discomfort intensifies. Ask for pain medication or anesthesia if you want it. Your health care team will partner with you to make the best choice for you and your baby. Remember, you're the only one who can judge your need for pain relief.

How long it lasts: Active labor often lasts 4 to 8 hours or more. On average, your cervix will dilate at approximately 1 cm an hour.

What you can do: Look to your labor partner and health care team for encouragement and support. Try breathing and relaxation techniques to relieve your discomfort. Use what you learned in childbirth class or ask your health care team for suggestions.

Unless you need to be in a specific position to allow for close monitoring of you and your baby, consider these ways to promote comfort during active labor:

Change positions

Roll on a large rubber ball (birthing ball)

Take a warm shower or bath

Take a walk, stopping to breathe through contractions

Have a gentle massage between contractions

If you need to have a Cesarean delivery (C-section), having food in your stomach can lead to complications. If your health care provider thinks you might need a C-section, he or she might recommend small amounts of clear liquids, such as water, ice chips, popsicles and juice, instead of solid foods.

The last part of active labor - often referred to as transition - can be particularly intense and painful. Contractions will come close together and can last 60 to 90 seconds. You'll experience pressure in your lower back and rectum. Tell your health care provider if you feel the urge to push.

If you want to push but you're not fully dilated, your health care provider will ask you to hold back. Pushing too soon could make you tired and cause your cervix to swell, which might delay delivery. Pant or blow your way through the contractions. Transition typically lasts 15 to 60 minutes.

Stage 2: The birth of your baby

It's time! You'll deliver your baby during the second stage of labor.

How long it lasts: It can take from a few minutes to a few hours or more to push your baby into the world. It might take longer for first-time moms and women who've had an epidural.

What you can do: Push! Your health care provider will ask you to bear down during each contraction or tell you when to push. Or you might be asked to push when you feel the urge to do so.

When it's time to push, you may experiment with different positions until you find one that feels best. You can push while squatting, sitting, kneeling - even on your hands and knees.

At some point, you might be asked to push more gently - or not at all. Slowing down gives your vaginal tissues time to stretch rather than tear. To stay motivated, you might ask if you could feel the baby's head between your legs or see it in a mirror.

After your baby's head is delivered, the rest of the baby's body will follow shortly. The baby's airway will be cleared if necessary. If you've had an uncomplicated delivery, your health care provider may wait a few seconds to a few minutes before the umbilical cord is cut. Delaying clamping and cutting the umbilical cord after delivery increases the flow of nutrient-rich blood from the cord and the placenta to the baby. This increases the baby's iron stores and reduces the risk of anemia, promoting healthy development and growth.

Stage 3: Delivery of the placenta

After your baby is born, you'll likely feel a great sense of relief. You might hold the baby in your arms or on your abdomen. Cherish the moment. But a lot is still happening. During the third stage of labor, you will deliver the placenta.

How long it lasts: The placenta is typically delivered in 30 minutes, but the process can last as long as an hour.

What you can do: Relax! By now your focus has likely shifted to your baby. You might be oblivious to what's going on around you. If you'd like, try breastfeeding your baby.

You'll continue to have mild, less painful contractions that are close together. The contractions help move the placenta into the birth canal. You'll be asked to push gently one more time to deliver the placenta. You might be given medication before or after the placenta is delivered to encourage uterine contractions and minimize bleeding.

Your health care provider will examine the placenta to make sure it's intact. Any remaining fragments must be removed from the uterus to prevent bleeding and infection. If you're interested, ask to see the placenta.

After you deliver the placenta, your uterus will continue to contract to help it return to its normal size.

A member of your health care team may massage your abdomen. This may help the uterus contract to decrease bleeding.

Your health care provider will also determine whether you need repair of any tears of your vaginal region. If you don't have anesthesia, you'll receive an injection of local anesthetic in the area to be stitched.

Now what?

Savor this special time with your baby. Your preparation, pain and effort have paid off. Revel in the miracle of birth.

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Show references

1. Funai EF, et al. Management of normal labor and delivery. https://www.uptodate.com/contents/search. Accessed Oct. 28, 2021.

2. Caughey AB, et al. Nonpharmacologic approaches to management of labor pain. https://www.uptodate.com/contents/search. Accessed Oct. 28, 2021.

3. Satin AJ. Labor: Diagnosis and management of the latent phase. https://www.uptodate.com/contents/search. Accessed Oct. 28, 2021.

4. American College of Obstetricians and Gynecologists. Labor and delivery. In: Your Pregnancy and Childbirth: Month to Month. Kindle edition. 7th ed. American College of Obstetricians and Gynecologists; 2021. Accessed Oct. 28, 2021.

5. Landon MB, et al. Normal labor and delivery. In: Gabbe's Obstetrics: Normal and Problem Pregnancies. 8th ed. Elsevier; 2021. http://www.clinicalkey.com. Accessed Oct. 28, 2021.

6. Meek JY, et al. The first feedings. In: The American Academy of Pediatrics New Mother's Guide to Breastfeeding. Kindle edition. 3rd ed. Bantam Books; 2017. Accessed Oct. 28, 2021.

7. Cunningham FG, et al. Normal labor. In: Williams Obstetrics. 25th ed. McGraw-Hill; 2018. https://www.accessmedicine.com. Accessed Oct. 28, 2021.

8. Wick MJ (expert opinion). Mayo Clinic. Dec. 6, 2021.

See more In-depth

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Stages of labor and birth: Baby, it's time!

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Find out what happens during the different stages of labor and birth.

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After a baby is born, its need for nurturing continues. Each child is born into a set of ecological systems (developed by Urie Bronfenbrenner, a Russian-born America psychologist) that helps the child develop and grow.

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Interactions with the 5 systems are the center of theory for a developing child. They include....

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The Microsystem is compiled of parents, siblings, and peers that the child has close direct contact. These relationships encompass a child's immediate surroundings.

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is how different relationships in a child's microsystem work together. For example, how a child's school and parents interact and work together has an indirect impact on the child.

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The exosystem includes a parent's workplace, the parent's friends, the neighborhood they live in, or mass media. These systems do not have direct involvement with a child but they still affect their development.

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And finally, the macrosystem. This system is the laws, economics, religious institutions and educational systems that fit into the larger cultural context of a child's life. These are broad influences that surround a child and affect their development.

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Birth is an everyday occurrence, but one of the most complex processes on earth. As a human life begins and progresses through life, it continues to be a subject of wonder and awe, and one worth our continued efforts to study and gain insight from.

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Public Broadcasting Service. (2001, November 21). Life's greatest miracle. PBS. Retrieved February 6, 2023, from https://www.pbs.org/wgbh/nova/video/lifes-greatest-miracle/

3 Weeks pregnant: Pregnancy week by Week. BabyCenter. Retrieved February 6, 2023, from https://www.babycenter.com/pregnancy/week-by-week/3-weeks-pregnant

Cafasso, J. (2017, November 14). The second trimester of pregnancy. Healthline. Retrieved February 6, 2023, from https://www.healthline.com/health/pregnancy/second-trimester

Editor. (2021, December 9). Third trimester of pregnancy: Fetal development. American Pregnancy Association. Retrieved February 6, 2023, from https://americanpregnancy.org/healthy-pregnancy/pregnancy-health-wellness/third-trimester/

Mayo Foundation for Medical Education and Research. (2022, January 13). Stages of labor and birth: Baby, it's time!Mayo Clinic. Retrieved February 6, 2023, from https://www.mayoclinic.org/healthy-lifestyle/labor-and-delivery/in-depth/stages-of-labor/art-20046545