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Second Trimester

Many women consider the second trimester (weeks 13 through 26) the best part of pregnancy. Here's why: The symptoms of the first few months have largely abated, and the discomforts of the last trimester have not yet started.

Now your pregnancy is visible for all the world to see! If you have not already started to wear maternity clothes, you'll likely need them now. Each week your developing baby-now officially a fetus-grows a bit more. Your practitioner will measure fetal growth at your prenatal visits, albeit indirectly. He or she will use a tape measure to determine the distance from your pubic bone to the top of the uterus (also known as the fundus). This measurement is known as the fundal height.

During the second trimester, the mother first notices fetal movement, generally some time between 16-20 weeks. However, reflexive movements begin during the embryonic period, and the fetus becomes active before the mother notices. Early in pregnancy, the fetal movements are relatively weak, and there is a greater amount of room available for movement. Therefore, it is unlikely that the fetus will bang up against the uterine wall with enough force for Mom to notice.

Experienced mothers remember the first flutterings from previous pregnancies and are more attuned to even the weakest movements. That's why they are more likely to notice fetal movement earlier. Later in pregnancy, the fetus is much stronger and relatively more confined. In fact, fetal movements become so vigorous they can be observed through the mother's abdominal wall. They have been known to keep expectant mothers up half the night-and expectant fathers, too, if the parents-to-be sleep too close together!

Vital organs continue to mature throughout the second trimester. Babies born before approximately 24 weeks are considered pre-viable (too young to survive). That's because their lungs are not sufficiently developed to exchange oxygen. The lungs gradually begin to acquire this critical capacity during the last several weeks of the second trimester. However, with appropriate support in a neonatal intensive care unit, most babies born after 26 weeks can survive. Babies born at this early age may weigh as little as two pounds and may have extremely serious medical problems.

The development of the external genitalia is completed early in the second trimester, and the baby's sex can usually be recognized by ultrasound at the same time that it is safe to perform amniocentesis. In females, the fetal ovary undergoes significant development during the second trimester. Remarkably, all the ova that the ovaries will ever contain are formed by the fifth month of fetal life! After their formation, the ova enter a state of rest in which they will remain until puberty. Nature prepares for the next generation even before the new generation is born!

Several important prenatal tests are administered during the second trimester. These include ultrasound, serum alphafetoprotein assay (AFP), and, possibly, amniocentesis.