Here a human egg cell 24 hours after fertilization is artificially colored purple. Around the egg is a thick layer (yellow) that has now become impenetrable. The two red areas, or pro-nuclei, contain genetic material from the mother and father before it has fused.
At this early stage following fertilization of your egg, the developing embryo will signal its existence to the pituitary gland in your brain and switch off your menstrual cycle. It does this by producing a new hormone, called human chorionic gonadotrophin (hCG). This hormone overrides your usual monthly cycle and maintains the high progesterone levels that are essential for your pregnancy. The hormone progesterone (see This is Day 4 of your Menstrual Cycle) is essential to an embryo's survival in the uterus, and therefore to your baby's well-being and development before birth.
Later, starting around weeks four to five, your embryo will make all the hormones needed to maintain its own life. Of course its nourishment and shelter come from you, but even in the very early weeks of pregnancy the embryo behaves like an independent human, at least as far as its hormones and genes are concerned.
When you're trying to conceive, you'll find you are much more aware of your general health. As a rule, colds, flu, and other common infections are unlikely to affect your fertility or your unborn baby if you have conceived. Some infections and viruses, however, can have a more serious impact:
Even if you don't appear to have ovulated this month, you may still have. It's possible to miss the LH surge just by chance. This is more likely if you don't test at the same time each day, or you drink a lot of water.
Remember too that ovulation tests are imperfect, and it's possible to get a false negative. If you had other symptoms of ovulation, such as pain, or changes in your mucus (see Changes during the menstrual cycle), it's likely that you ovulated anyway. However if you have gone two or three months with consistently negative tests, then you might not be ovulating regularly. In that case, it's worth seeking medical advice.
Too much testosterone can affect a woman's fertility.
Small quantities of testosterone are secreted from the adrenal gland and the ovaries. In low levels it may aid fertility, but too much can affect the menstrual cycle and lead to infertility.
Excerpted from Pregnancy Day by Day.
Copyright © 2008 Dorling Kindersley Limited.
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