From the Guidelines
Alpha-Fetoprotein (AFP) screening is used at 18 weeks gestation to assess the risk of neural tube defects and certain chromosomal abnormalities in the developing fetus, as it is the optimal time for screening, with AFP levels peaking between 16-18 weeks, as stated in the study by 1 and further supported by 1 and 1. The 18-week timeframe allows for the most reliable baseline for comparison, as AFP levels naturally rise during pregnancy, and provides enough time for follow-up diagnostic testing if abnormal results are detected. This timing also enables parents to make informed decisions about their pregnancy. Key factors that influence the accuracy of AFP screening include gestational age, maternal weight, race, presence of insulin-dependent diabetes, number of fetuses, and family history of neural tube defects, as highlighted in the study by 1. It is essential to note that AFP screening is not diagnostic but rather a risk assessment tool that may indicate the need for more definitive testing, such as ultrasound or amniocentesis, as emphasized in the study by 1. Some of the benefits of AFP screening at 18 weeks gestation include:
- Detection of neural tube defects, such as spina bifida or anencephaly
- Identification of chromosomal disorders, like Down syndrome (trisomy 21)
- Opportunity for genetic counseling and additional testing
- Informed decision-making for parents regarding their pregnancy The study by 1 recommends that women be offered MSAFP screening for the detection of open neural tube defects and anencephaly, optimally between 16 to 18 weeks gestation, and that multiple marker screening (AFP, hCG, uE3 with or without INH-A) should be offered to women unless an amniocentesis is indicated based on their history and/or age. Overall, AFP screening at 18 weeks gestation is a crucial tool for assessing the risk of neural tube defects and chromosomal abnormalities, and its optimal timing is supported by the studies by 1, 1, and 1.
From the Research
Alpha-Fetoprotein (AFP) Screening
- AFP screening is used at 18 weeks gestation to detect neural tube defects (NTDs) in fetuses 2.
- The best time for detecting open spina bifida by measuring maternal serum-AFP is at 16-18 weeks of pregnancy 2.
- At 16-18 weeks of pregnancy, 88% of cases of anencephaly, 79% of cases of open spina bifida, and 3% of unaffected singleton pregnancies had AFP levels equal to or greater than 2.5 times the median for unaffected singleton pregnancies 2.
Detection of Neural Tube Defects
- AFP screening can detect NTDs, including anencephaly and spina bifida, with a detection rate of 50.0% at a fixed false-positive rate of 10% 3.
- The predictive value of an elevated AFP varies with the population screened and is reduced by routine ultrasonography before amniocentesis 4.
- In a low-risk population, an elevated AFP is most often not associated with a fetal neural tube defect 4.
Screening Methods
- Maternal serum-AFP measurement is an effective method of selecting women for ultrasonography and amniocentesis to diagnose NTDs in utero 2.
- First-trimester screening for NTDs using alpha-fetoprotein may be useful in screening for fetal NTDs 3.
- Imaging, such as high-quality ultrasound, may replace AFP assays for the detection of fetal NTDs in the near future 5.