Now that you have made the decision to become pregnant, the next nine months may be full of second thoughts, misgivings, and increased stress. The length of a full-term pregnancy is 40 weeks, counted from the last menstrual period an base on a typical 28-day menstrual cycle. Because women's cycles can vary from 21 days to 35 days, knowing the typical length of your cycle will help you and your physician or certified nurse midwife (CNM) to plan prenatal testing during your pregnancy.
Your care provider may have already performed tests to rule out risk factors, including: anemia, sexually transmitted diseases, urinary-tract infections, susceptibility to certain viral infections (including German measles and hepatitis), and vaginal infections that are associated with pre-term labor. If these tests were not done before you became pregnant, they may be performed at your initial pregnancy visit. You should also be tested for the human immunodeficiency virus (HIV) that causes AIDS. By law, your consent is required before this test can be performed and you are the only one to whom the results may be disclosed.
If there is an increased risk for chromosomal or genetic abnormalities, chorionic villus sampling (CVS) may be performed at ten to 12 weeks of pregnancy. This procedure is invasive (involving removal of a small piece of the placenta for examination), and should be done by a trained obstetric specialist; even so, complications in one to two percent of these cases lead to failure of the pregnancy. Genetic amniocentesis, another invasive diagnostic procedure, is performed at approximately 16 weeks of pregnancy. Amniocentesis has only 25 to 50 percent as many serious complications as CVS.
Perhaps the most important prenatal tests are ultrasound imaging studies. Although these studies are best done at the midpoint of pregnancy, earlier imaging may be appropriate in some cases. You and your care provider should discuss where these studies should be performed. In many cases, the services of a regional perinatal care center will be chosen so as to take advantage of the combination of experienced specialists in fetal imaging (sonologists), maternal-fetal medicine, and genetics.
These specialists can perform and interpret a "targeted" ultrasound study which looks for: abnormalities in the head and neck anatomy, spine, heart, chest and diaphragm, stomach, kidneys and bladder, abdominal wall and umbilical cord, upper and lower extremities, the placenta, and amniotic fluid volume. …