At a follow-up routine prenatal visit, the uterine fundus of a healthy 23-year-old pregnant woman is palpated halfway between her symphysis pubis and umbilicus. Which of the following is the most appropriate test to order at this stage of her pregnancy?
- serum human immunodeficiency virus (HIV) titer
- glucose tolerance test
- amniocentesis
- maternal serum alpha-fetoprotein (MSAFP)
- cervical culture for group B Streptococcus (GBS)
Answer(s): D
Explanation:
The fundal height corresponds to 16 gestational weeks. Between 15 and 20 weeks, screening for open neural tube defects should be offered. In addition to MSAFP, the American College of Obstetricians and Gynecologists recommends hCG and unconjugated estriol to screen for Down syndrome and trisomy 18 as well. This triad of tests is called a triple screen or triple marker screen. Reported sensitivity of the triple screen is between 57 and 67% and the false positive rate is 5%. An abnormal result must be evaluated further by ultrasonography to identify the presence or absence of open neural tube defects or abdominal wall defects (increased MSAFP) or trisomy disorder (decreased MSAFP and unconjugated estriol, increased hCG). In skilled hands, an ultrasound reduces the risk of such an anomaly by 95%. If the diagnosis is still uncertain, the woman should be offered amniocentesis for measurement of alpha- fetoprotein
(AFP) and acetylcholinesterase activity (increased in neural tube defects) and karyotype of fetal skin cells.
Although testing for HIV can be done any time, it is most appropriate at the first prenatal visit, because earlier onset of prophylaxis with acquired immune deficiency syndrome (AIDS) drugs reduces the risk of transmission to the fetus significantly. Routine culture for GBS is not recommended because of the high recurrence rate after treatment and the low attack rate to the fetus. Amniocentesis is not a screening procedure and is reserved for those women with a specific indication, such as elevated MSAFP, low MSAFP (risk of Down syndrome), advanced maternal age, and others. A glucose tolerance test may be appropriate if there is a clinical indication for diabetes mellitus: previous macrosomic infant or stillbirth, strong family history of diabetes mellitus, persistent glycosuria, previous gestational diabetes, or elevated random serum glucose concentration.