Maternal Serum Screening
You can specify which conditions you wish to test for, including Down syndrome, trisomy 18, or an open neural tube defect such as spina bifida. Please keep in mind that this test will never tell you for sure if your baby has one of these conditions.
This test is offered to all women who are pregnant, usually between 15 and 20 weeks of pregnancy. This screening measures your blood for the level of a protein and certain hormones made by your baby. It also looks at other factors such as the fetus's weight and age, whether you are having one or two babies, and your age.
Knowing how far along you are in the pregnancy is important because the level of the protein and these hormones changes each week of the pregnancy. Incorrect pregnancy dates can result in a false negative or false positive test result. Your age also plays a role in whether this test is more likely to come back positive.
A positive test for Down syndrome or trisomy 18 may indicate that your chances of having a baby with one of these conditions may be greater than someone your age, depending on your age-related risk before testing.
A positive test for a neural tube defect may indicate an increased risk for a baby with:
- Spina bifida, which leaves part of the spine exposed outside the body
- Anencephaly, which indicates that the skull did not grow closed over the brain
- Gastroschisis or omphalocele, which leave part of the intestines exposed outside the body
Depending on the condition and its severity, a baby with any of these conditions may be able to have surgery and lead a normal life.
If you have a positive screening
- Again keep in mind that having a positive test result does not mean your baby has one of these conditions. The results of the screening test are used to determine if further testing is recommended.
- You should consider having a detailed ultrasound to examine the baby's growth and development. If the ultrasound does not show any signs of these conditions, their risk will be smaller.
- If you choose to, you can have an amniocentesis, a very accurate test that looks at the baby's chromosomes and a protein in the amniotic fluid to tell you with more certainty whether the baby has one of these conditions. This is usually done between 15 and 20 weeks of pregnancy.
- Regarding a positive screen for spina bifida, if you are less than 18 weeks, your gestational date has been confirmed and the AFP levels are not very high and your doctor may offer to repeat the blood test (at least a week later). If the AFP returns to a normal level, then the first result was more likely a false positive.
- Depending on your test results, another ultrasound at 32 weeks may be recommended to see if your baby is growing well.
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