The embryonic phase of human development lasts from two weeks after conception--during the first two weeks, the fertilized egg is referred to as a zygote--until the end of the eighth week of pregnancy, at which time the embryo becomes a fetus. While organs are not completely functional by the end of the embryonic phase, the embryo nevertheless possesses all the parts of a fully developed human in the eighth week, making the embryonic phase on of the busiest times in human development.
Week Five
American Pregnancy notes that pregnancy math can actually be quite confusing--the beginning of the third week after conception, which is also the beginning of the embryonic phase, is actually the beginning of the fifth week of pregnancy. This is because pregnancy length is calculated from the first day of a woman's last menstrual period, meaning that during pregnancy weeks one and two, she has not yet ovulated. By week five, the zygote has become an embryo with a beating heart, and the beginnings of eyes and ears. Still unable to see or hear, these early sensory structures gain function in the coming weeks.
Week Six
In her book "Conception, Pregnancy and Birth," Miriam Stoppard, M.D. notes that while a sixth week embryo is still not quite human in appearance, it's nevertheless forming human structures rapidly. The spinal cord and brain develop, as do the bones that will eventually become the jaw. Buds on rudimentary hands and feet represent the beginnings of what will be fingers and toes.
Week Seven
By the end of week seven, the embryo has reached another important developmental milestone--all of the essential human organs have formed, though they are not all functional just yet. The embryo is nearly half an inch long by this time, and is becoming less tadpole-like in appearance as its spine straightens and lengthens. In the bestselling pregnancy book, "What to Expect When You're Expecting," authors Heidi Murkoff and Sharon Mazel note that brain development continues at a staggering rate during the seventh week--embryos at this stage generate 100 new brain cells each minute.
Week Eight
The eighth week marks the last week of the embryonic phase, and an important milestone in human development. The skeletal system continues to develop, though most bones are still cartilaginous in nature, and early muscles begin to contract. American Pregnancy notes that the last vestiges of the embryo's resemblance to a tadpole fade with the disappearance of its tail. A clearly human face emerges as facial bone structure continues to develop, and the genitals begin to form for the first time, though gender was determined at the moment of conception. By the end of the embryonic period, the baby is visibly and distinctly human, though still quite tiny--about the size of a raspberry, according to Murkoff and Mazel.
References
- American Pregnancy: Week five
- "Conception, Pregnancy and Birth;" Miriam Stoppard, M.D.; 2008
- "What to Expect When You're Expecting;" Heidi Murkoff and Sharon Mazel; 2008
- American Pregnancy: Week eight
