The goal of fertility treatment is to achieve pregnancy. Once a sperm and egg unite to form a zygote, the developmental progression that eventually results in a human baby is nothing short of miraculous.

Pregnancy is defined as beginning when a fertilized zygote becomes implanted in a woman's uterus. This occurs when the embryo becomes embedded into the endometrium (lining of the uterus) where it forms a placenta to receive essential nutrients which pass through the uterine wall. The zygote undergoes rapid divisions with no significant growth (a process known as cleavage) and cellular differentiation, leading to the development of an embryo. The term embryo refers to the early stages of development, after the zygote has divided at least once. In humans, the term is used to describe the first two months of development starting at 3 weeks and continuing until the eighth week of pregnancy. From the end of the eighth week until its birth the developing organism is referred to as a fetus.

Day one after fertilization, the zygote proceeds immediately to the first cleavage (two-cells). Subsequent cell divisions follow rapidly; four cells two days after fertilization; eight cells three days after fertilization. The cells are called blastomeres, and the organism as a whole is called a morula from the time it has 16 blastomeres to the next stage. The morula is still encased in the zona pellucida (the membrane surrounding the oocyte). As it is undergoing this very rapid cell division, the organism is also moving down the uterine tube toward the uterus. After it arrives in the uterus, around day five after fertilization, the zona pellucida breaks up; a process called “hatching” necessary for implantation.

When the morula enters the uterus, fluid starts to accumulate between its blastomeres. The fluid-filled spaces run together, forming a relatively large fluid-filled cavity. At the point when the cavity becomes recognizable, the organism is called a blastocyst. The outer cells of the blastocyst, especially those around the blastocyst cavity, assume a flattened shape. The flattened cells of the exterior blastocyst are the trophoblast. They become the placenta and other supporting structures. On one side of the blastocyst is a group of cells that project inside into the blastocyst cavity; this is called the inner cell mass, which will become the body of the baby.

After the embryo covering degenerates, the blastocyst, now in the uterus, enlarges. Its trophoblast (invasive, eroding, and metastasizing cells of the placenta that are formed during the first stage of pregnancy) attaches to the endometrium (uterine lining) around six days after fertilization. This begins the process of implantation, during which the blastocyst becomes integrated with the endometrium through specialized membranes. The embryo is now beginning its second week of development. The process of implantation takes three to four days, and is generally completed by day twelve. As the blastocyst becomes more deeply embedded in the endometrium, the layered area expands until finally the whole trophoblast surface has divided into one layer or the other. The inner cell mass then separates itself from the overlying trophoblast. The resulting space is called the amniotic cavity and the layer of cells that forms its roof is called the amnion. The amniotic sac is a thin transparent pair of membranes, which hold a developing embryo (and later fetus) until shortly before birth. The inner membrane, the amnion, contains the amniotic fluid and the fetus. The outer membrane, the chorion, contains the amnion.

Outgrowths of trophoblast from the chorion project into the endometrium and later give rise to the placenta. Although the blastocyst has become completely embedded in the endometrium maternal blood does not enter the blastocyst. Later, as the fetal circulation develops, the fetal and maternal blood systems still remain separate and do not mix together. Nutrients, oxygen, and wastes diffuse in the appropriate direction across the placenta, but the two blood systems are individual and do not combine.

If you are pregnant and would like to learn more about the developing fetus/pregnancy there are numerous resources available in print and on the Internet. Below is a list of websites and links.

WebMD: http://www.webmd.com/pregnancy_and_family/default.htm

American Pregnancy Association: http://www.americanpregnancy.org/duringpregnancy/index.htm

The Merck Manuals Online: http://www.merck.com/mmhe/sec22/ch257/ch257c.html

Finally, the book A Child Is Born by Lennart Nilsson is an interesting pictorial resource and it’s available in most bookstores and on the Internet.

References:
1. Embryogenesis from Wikipedia. Available at: http://en.wikipedia.org/wiki/Embryogenesis

2. Monitoring stem cell research. Appendix A: Notes on Early Human Development. Available at: http://bioethicsprint.bioethics.gov/reports/stemcell/appendix_a.html