Second Trimester Prenatal Screening
The risk of producing a child with Down syndrome or an open neural tube defect concerns many parents. This risk can be assessed by screening the level of alpha-fetoprotein (AFP) in the mother’s bloodstream, which can be measured between the 15th and 21st week of pregnancy.
Open neural tube defects
AFP screening can detect pregnancies with open neural tube defects because these defects allow greater than normal amounts of AFP to cross into the mother’s bloodstream. There are two types of open neural tube defects – an absence of the skull, known as anencephaly, and an open spine, known as spina bifida. These defects generally occur without warning and without an affected family member.
Most women who have AFP testing will have normal results, meaning that their risk of having a baby with anencephaly or spina bifida is very low. The screening detects 80 percent of pregnancies with open neural tube defects.
If the AFP level is higher than expected, the possibility of twins or incorrect dating (being further along in pregnancy than expected) must first be ruled out by an ultrasound. A repeat AFP blood test is usually performed if neither twins nor incorrect dating is found by the ultrasound.
If a woman has elevated AFP levels on repeat testing or the first AFP level is very high, she will be offered further information and testing to evaluate the baby for abnormalities. IT’S IMPORTANT TO NOTE, MOST WOMEN WHO HAVE ELEVATED AFP LEVELS WILL HAVE HEALTHY BABIES. Elevations usually can be explained by the differences in placentas and the amount of AFP that is allowed to cross into the mother’s bloodstream.
The AFP level and other blood levels also are used to detect pregnancies at increased risk for Down syndrome, a birth defect resulting in mental retardation. Down syndrome, like open neural tube defects, generally occurs without warning or without an affected family member. The risk of having a baby with Down syndrome increases as women age. While it is routine to offer prenatal testing, such as amniocentesis, to all women 35 years of age and older, Down syndrome pregnancies in women under 35 generally remain undetected until delivery unless a blood screening has been done.
Low AFP, hCG, and estriol levels are used to detect pregnancies at increased risk for trisomy 18, a birth defect that causes mental retardation, and in most cases, death. This screening will detect 60 percent of pregnancies with trisomy 18.
If you have questions about screening or diagnostic tests in pregnancy, please call the MUSC Prenatal Wellness Center at 843-792-1212.
What you need to know:
Prenatal tests are the tests you and your baby receive while you are pregnant. They allow health care providers to learn about how a baby is doing before birth, as well as how Mom is doing during her pregnancy.
Some prenatal tests, such as blood tests, urine tests and blood pressure checks are routine—meaning all pregnant women get them. Others, such as amniocentesis, are not offered routinely, but only if you or your baby is at increased risk of certain problems or conditions.
What you can do:
Gather as much information as you can about any prenatal test your health care provider offers you. Talk to your partner, other family members, a spiritual advisor and/or genetic counselor if you are uncertain about whether or not you should have certain prenatal tests. Remember, most women get reassuring news that the baby is just fine.
Source: March of Dimes Birth Defects Foundation