Your Baby's First Trimester Development


First Signs You're Pregnant
At about this time (three weeks from conception, five weeks from the start of the last period), the mother's breasts begin to feel very sensitive and tender. This is often the first symptom of pregnancy. The queasy stomach, nausea and vomiting typically begin the following week. When a woman is a few days (or hours in some cases!) late, the pregnancy test may be positive. Some test kits are sensitive enough to pick up small amounts of hCG (human chorionic gonadotropin), a hormone secreted by the cells that are burrowing into the endometrial lining of the uterus. By six weeks from the last menstrual period, four weeks from conception, all pregnancy tests should be positive if the pregnancy is developing appropriately.

Among the first organs to develop, the nervous system is represented by the "neural groove," which makes its appearance now. One month after conception, the embryo is about four millimeters long and takes on a "C" shape. Early arm and leg buds appear, and there is rapid cellular development of every organ system. The placenta is functional now, and there is a beginning exchange of blood from mother to baby. A baby at this stage becomes more and more vulnerable to disease and infection as well as alcohol, substance abuse and some medications. The eyes and ears are undergoing rapid and precise development now. If the mother should contract a rubella infection at this time, the risk for eye and ear damage would be approximately 81 percent, whereas it would decrease to 25 percent if she were infected at the end of the second trimester.


By six weeks from conception, other developmental milestones occur as circulation begins and a tube that will become the heart pulsates. At one month past fertilization, the baby has a mouth and a rudimentary tongue. Baby is now one-tenth of an inch long.

At eight weeks from conception '- the last stage of embryonic development '- the embryo has completed development of all internal and external structures and takes on a humanlike appearance. The tail is gone and the baby can move, but the movements are not yet able to be felt by the mother. Prenatal care should start at six to eight weeks, and by 10 weeks the fetal heart should be heard with an office Doppler device (unless the uterus is tipped back or the mother has a high body mass index). Baby is now about 1 1/4 inches long. The placenta and embryo are about the size of a golf ball.

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