Furthermore, the embryo is fixed in the upper part of the uterus and surrounded by the gestational sac, which protects the baby from infection or trauma and is responsible for forming the placenta and amniotic sac.

In the fourth week of gestation, three layers of cells have already formed that will start all of the baby's organs and which grow lengthwise, giving the embryo a more elongated shape. In addition, the umbilical cord starts to form and the transport of nutrients and oxygen to the embryo begins.

The outermost layer of cells, the ectoderm, forms the baby's neural tube, skin, hair, nails, and teeth, and the intermediate layer, known as the mesoderm, forms the heart, blood vessels, bones, muscles, and reproductive organs. The innermost layer of cells, the endoderm, will form the lungs, liver, bladder and digestive system.

At approximately 27 days of gestation, which is equivalent to the penultimate day of the fourth week, the baby already has its spinal column and spinal cord formed, from the cervical to the coccyx.

The embryo's heart beats fast and the features that will give rise to the baby's face begin to develop. The embryo's tiny head shows two dark spots, which will give rise to its eyes, and dimples that will form its ears and nostrils. In addition, small folds of tissue below the developing brain will become the baby's tongue, jaw, chin, and neck.

Muscle and bone tissue are forming covered by a thin layer of skin, arms and legs are developing, appearing as tiny sprouts, and the spinal column extends giving the appearance of a small tail that will disappear in a few weeks.

During this phase, the lungs, kidneys, liver and other organs of the digestive, reproductive and urinary systems also begin to form.