Maternal serum alpha-fetoprotein (MSAFP)
Screening at 15-20 weeks gestation, is optimal at 18-18 weeks gestation. Alpha fetoprotein (AFP) screening during pregnancy has had a major impact on the prenatal detection of fetal abnormalities. Elevated MSAFP levels correlate with an increased risk of Neural tube defects (NTD), whereas low MSAFP levels are suggestive of an increased risk of fetal Down syndrome. It is important to begin with the use of MSAFP as a standard test. All patients should have the MSAFP test performed, or sign a waiver stating they reject the administration of the MSAFP test on themselves. The majority of clinical values are associated with a normal fetus. When a defects are present, half are NTDs and half are abdominal wall defects.
Open neural tube defects (NTD) are virtually always associated with elevated levels of amniotic fluid AFP and the presence of acetylcholinesterase, but they are not always associated with elevated levels of MSAFP. Closed NTDs, including those associated with hydrocephalus, are not associated with abnormal AFP findings. Elevated MSAFP levels also exist in multiple pregnancies and certain fetal abnormalities (e.g., omphalocele, congenital nephrosis, Turner syndrome with cystic hygroma, fetal bowel obstruction, teratoma). Moreover, small-for-date fetuses and fetal death may be associated with high levels of AFP in amniotic fluid or maternal serum. Incorrect assessment of gestational age may create falsely high or low MSAFP levels.
Patients with a personal or first-degree family history of NTD should be advised of the risk of having an affected fetus. Because MSAFP screening will detect only 70 percent to 80 percent of open NTDs, these patients should be offered amniocentesis at 15-16 weeks of gestation with amniotic fluid AFP testing. Ultrasound evaluation of the fetus for NTD at 16-18 weeks of gestation can add further information. In patients with no history of ND (i.e., most of the obstetric population), it is now accepted practice to offer the MSAFP screening at 15-20 weeks of gestation. Maximal accuracy requires that the initial sample be obtained at 16-18 weeks of gestation. MSAFP levels are elevated (2.5 multiples of the median (MOM) in 80 percent to 90 percent of pregnancies in which the fetus has an NTD. Because considerable overlap exists between the MSAFP level in normal pregnancy and the MSAFP level characterized by a fetus with NTD, false-negative and false-positive values are inevitable. Thus, sequential protocols for distinguishing the reason for an elevated MSAFP level other than NTD are needed. Either a second MSAFP test, performed within one to two weeks of the time of the first sample, or an ultrasound evaluation without a repeat of the MSAFP test may be performed, depending on the protocol of the laboratory.
Pregnancies with fetal Down syndrome may be associated with low MSAFP levels. A more accurate estimate of the risk of fetal Down syndrome can be made by using a combination of MSAFP level and maternal age. For example, in a 25-year-old woman the risk of delivering an infant with Down syndrome, based on age alone, is one in 1,250. If her MSAFP level is 0.36 MOM at 16 weeks of gestation, however, the risk is increased to one in 257. The benefit of amniocentesis in such a couple may then outweigh the low yet finite risk of the procedure. When a protocol is used in women under age 35 years, 20 percent to 30 percent of fetuses with Down syndrome can be identified by performing amniocentesis in two percent to four percent of women. Obstetricians should consult their local provider of genetic diagnostic services concerning the implementation of MSAFP screening and interpretation of results for this purpose. The current practice of offering prenatal diagnosis of the detection of chromosomal abnormalities based on advanced maternal age alone (i.e., 35 years of age or older at the EDD) should continue irrespective of the MSAFP level. In addition to low MSAFP levels, other maternal serum markers proposed to aid in the screening for fetal Down syndrome include elevated levels of the hCG and low levels of unconjugated estriol.
The following screening tests should be performed in endemic areas or for women with risk factors:
An educational program at the preconception and/or first pregnancy visit is appropriate. Screening by serology at the first pregnancy visit may be appropriate, but only for those known to be at risk (i.e., those who have a new or "outside" cat or who eat raw meat). A retest should be done at 16 to 20 weeks. Converters may be referred to a tertiary center for percutaneous fetal blood sampling and culturing, and, if positive, treatment versus termination. Because of the high prevalence of maternal infection and the seriousness of prenatal transmission, routine preconception screening is recommended. The presence of antibodies provides reassurance about immunity. The absence of antibodies underscores the need for education.
Persons at high risk for tuberculosis include: Persons with HIV infection and those at risk for HIV, close contacts of infectious tuberculosis cases, persons with medical conditions which increase the risk of tuberculosis, foreign-born persons from high prevalence countries, low-income populations (including high-risk minorities), alcoholics and intravenous drug users, residents of long-term care facilities (including prisons), and the homeless. Screening by skin test should be performed only in women at high risk for the disease or in high-risk populations or in endemic areas.
At the preconception visit or during pregnancy, obtain a history of herpes lesions. If lesions are present, a single culture to confirm the diagnosis may be indicated. Routine prenatal screening by culture is not indicated. Examination of the cervix and vagina for lesions in early labor should be performed.
Routine pregnancy screening is not indicated. During pregnancy, titers to determine immunity to infection may be useful for counseling, in the event of exposure to a woman lacking a history of chickenpox. Treatment of the exposed seronegative mother with varicella-zoster immune globulin is controversial, but recommended by some authorities. Additional research is warranted. In the event of maternal chick pox or herpes-zoster, appropriate counseling about risk of congenital varicella syndrome should be performed. Appropriate treatment of the newborn with recent active chick pox with varicella-zoster immune globulin is indicated. The pending release of varicella vaccine raises the potential for vaccination of susceptible women prior to conception.