- Review the stages of prenatal development.
- Explain how the developing embryo and fetus may be harmed by the presence of teratogens and describe what a mother can do to reduce her risk.
Conception occurs when an egg from the mother is fertilized by a sperm from the father. In humans, the conception process begins with ovulation, when an ovum, or egg (the largest cell in the human body), which has been stored in one of the mother’s two ovaries, matures and is released into the fallopian tube. Ovulation occurs about halfway through the woman’s menstrual cycle a nd is aided by the release of a complex combination of hormones. In addition to helping the egg mature, the hormones also cause the lining of the uterus to grow thicker and more suitable for implantation of a fertilized egg.
If the woman has had sexual intercourse within 1 or 2 days of the egg’s maturation, one of the up to 500 million sperm deposited by the man’s ejaculation, which are traveling up the fallopian tube, may fertilize the egg. Although few of the sperm are able to make the long journey, some of the strongest swimmers succeed in meeting the egg. As the sperm reach the egg in the fallopian tube, they release enzymes that attack the outer jellylike protective coating of the egg, each trying to be the first to enter. As soon as one of the millions of sperm enters the egg’s coating, the egg immediately responds by both blocking out all other challengers and at the same time pulling in the single successful sperm.