GotoMyDoc - Pregnancy - Baby Development - Learn

What It's About | Did You Know? | Learn More About | For More Info

Baby's development

Month 1

This is the most rapid and fragile growth period of pregnancy. The fertilized egg, called a zygote, divides approximately 18-36 hours after fertilization to become a two-celled organism. By 48-72 hours, the zygote becomes a morula, consisting of 16 cells. While this growth is occurring, the cell cluster is moving through the fallopian tube to the uterus. In about four days, the morula, now 32 cells, reaches the uterus.

Fluid begins to fill the hollow sphere that has been created. This sphere, a blastocyst, consists of several layers of cells and floats for two to four days until attaching to the uterine wall. The outer layer of cells, trophoblasts, become the placenta. These cells attach to the uterine wall around day 7. The inner layer of cells becomes the embryo. The remaining space inside the blastocyst is filled with a fluid that becomes the “bag of water” or amniotic fluid.

Next, placenta and the maternal blood supply to the placenta begin to develop. The embryo becomes a two-layer structure, the endoderm and the ectoderm. The chorion, an outer membrane, is the foundation of a third layer, the mesoderm. The amnion cavity is established and a yolk sac develops. The yolk sac will develop into the baby’s liver and digestive system.

By day 15-16 the three layers are complete. The three “germ” (root of the word germinate – to sprout, develop or grow from) layers, ectoderm, mesoderm, and endoderm, are the layers from which all the body systems are specifically derived.

Part of the ectoderm grows inward to form the foundation for the nervous system during weeks three and four. The rest of the ectoderm later forms the outer integumentary system (hair, skin, nails) and some of the endocrine system (glands that secrete hormones – like thyroid).

During the third week, the mesoderm lays the foundation for the muscular system, skeletal system, cardiovascular system, lymphatic system and parts of the integumentary (skin, hair, nails), endocrine, respiratory, urinary and reproductive systems.

At the same time, the endoderm or inner layer is laying the foundation for the digestive system (food tube, stomach and bowel) and parts of the reproductive, urinary, respiratory, and endocrine systems.

The single chamber heart begins to beat by day 25. By day 28, the neural tube (spinal column) of the nervous system should be completely closed. The thyroid begins to develop and early muscle cells are formed. The gut begins to differentiate into the separate functioning parts of the digestive system (food tube, stomach and bowel). The beginning of the limb buds, lung buds, tongue bud, parathyroid, lens, thymus, the urinary and reproductive systems also start to develop.

As you can see, so much occurs in the first month, as the foundation is established for further development in the months to come. The embryo is only about 1/2 of an inch long and weighs about an ounce…it is truly amazing!