Conception to Birth Physical Development
Geminal Period • Conception occurs when sperm successfully fertilizes an ovum. From there the Germinal Period begins and lasts two weeks. • The fertilized ovum is now known as a zygote. This period is characterized by rapid cell division, the beginning of cell differentiation, and implantation of the zygote into the uterine wall. • Implantation usually occurs ten days after conception and if it does not occur, the zygote is flushed out of the woman’s system in her menstrual cycle. • Not every zygote will implant, so medically speaking a woman becomes pregnant when implantation occurs. By the time implantation occurs the zygote is composed of 100 to 150 cells.
Embryonic Period • The embryonic period is the period of prenatal development that occurs from two weeks to eight weeks after conception. The zygote is now called an embryo. • The embryo’s endoderm, the inner layer of cells, primarily produces internal body parts such as the respiratory and digestive systems. • The middle layer of cells, called the mesoderm, will become the circulatory system, bones, muscle, excretory system, and reproductive system. • The outer layer of cells, the ectoderm, will become the nervous system, the sensory receptors (eyes, nose, and ears), and skin. • As the embryo develops so too does the placenta, the umbilical cord and the amnion (fluid filled sac). It is at the end of this period and the beginning of the fetal period that most women become aware that they are pregnant as they miss a menstrual cycle.
Fetal Period • The fetal period is the period of development that begins two months after conception and lasts for seven months on average. The embryo is now called the fetus. • Three months after conception the fetus is about 10cm long and weighs 60g (0.13lbs or 2.11 oz). The fetus becomes active, moving its arms and legs, opening and closing its mouth, and moving its head. The face, forehead, eyelids, nose, and chin are all distinguishable, as are the arms, hands, and legs. • At four months, a growth spurt occurs in the fetus’s lower parts. By the end of the fifth month the toenails and fingernails have formed and the fetus is more active. • By the end of the sixth month the eyes and eyelids have completely formed, a grasping reflex is present and irregular breathing occurs. In the last two months, fatty tissues develop and the functioning of organs such as the heart and kidneys steps up.
Viability • Pregnancy lasts 40 weeks and at 26 weeks the fetus is considered viable, meaning the fetus could survive outside of the uterus, but it is very risky.
As you can see, all the critical period occurs during the first trimester (conception to 12 weeks, about three months). The first trimester is also the highest risk for miscarriage, as there is a correlation that if something goes wrong during these critical periods, the fetus will miscarry. • Physical development will be influenced by genetic and environmental conditions. But more on that later…