What does high alpha-fetoprotein level mean in second trimester screening?

[Anonymous]

My second trimester screening showed a high alpha-fetoprotein level or another unusual marker. What could it mean for my baby?

[Genetic Counselor]

A high alpha-fetoprotein does not mean the baby has a problem. The most common reasons are simple ones: the pregnancy is further along than thought, there are twins, or some of the baby’s blood has mixed with the mother’s. The main concern it screens for is an open neural tube defect. The American College of Medical Genetics and Genomics reports that alpha-fetoprotein screening picks up about 95 in every 100 babies with anencephaly and about 65 to 80 in every 100 with open spina bifida. Other causes include an opening in the baby’s belly wall and, rarely, congenital nephrotic syndrome, an inherited kidney condition caused by changes in genes such as NPHS1.

The next step is a detailed, targeted ultrasound by an experienced fetal medicine specialist. This finds most of these conditions directly. If the scan is unclear, doctors may do an amniocentesis and test the fluid for alpha-fetoprotein and an enzyme called acetylcholinesterase.
When the scan is normal, the high level is called “unexplained”. Most of these babies are healthy. There is a slightly higher chance of placenta-related problems later, such as high blood pressure in pregnancy, slow growth, or early birth, so extra growth scans are often advised.

A very low unconjugated estriol can, rarely, point to specific genetic conditions:
• Smith-Lemli-Opitz syndrome: An inherited condition in which the body cannot make enough cholesterol, caused by changes in both copies of the DHCR7 gene. Both parents are usually healthy carriers, with a 1-in-4 chance in each pregnancy. Amniotic fluid testing can confirm it.
• Steroid sulfatase deficiency: A condition affecting boys, caused by changes in the STS gene on the X chromosome. It mainly causes dry, scaly skin (X-linked ichthyosis) after birth, and can be linked with a slow start to labour.
Neural tube defects themselves are usually not caused by a single gene. They result from a mix of genetic and environmental factors, including low folate. After one affected pregnancy, the chance of another is about 2 to 3 in every 100. Taking a higher dose of folic acid, as prescribed by your doctor, before the next pregnancy lowers this chance substantially.

[Author]

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