Second Trimester Tests During Pregnancy

Here are the prenatal tests that may be performed in the second trimester of your pregnancy:

Maternal serum alpha-fetoprotein (MSAFP) and multiple marker screening: One or the other is offered routinely in the second trimester. This test is an optional genetic screening test and as with all screening tests, talk with your doctor about the pros and cons to see if it is right for you. The MSAFP test measures the level of alpha-fetoprotein, a protein produced by the fetus. Abnormal levels indicate the possibility (but not existence) of Down syndrome or a neural tube defect such as spina bifida, which can then be confirmed by ultrasound or amniocentesis.

When blood drawn for the MSAFP test is also used to check levels of the hormones estriol and hCG, it's called the triple test. When a marker called inhibin-A is added to the screen, it is known as the quad marker. The quad marker test significantly boosts the detection rates for Down syndrome. The test picks up about 75% of neural tube defects and 75%-90% of Down syndrome cases (depending on the mother's age), but many women will have a false-positive screening. About 3% to 5% of women who have the screening test will have an abnormal reading, but only a small percentage of those women will actually have a child with a genetic problem.

Non-Invasive Prenatal Testing (NIPT) screening: This cell-free fetal DNA test can be done as early as after 10 weeks of pregnancy. The test uses a blood sample to measure the relative amount of free fetal DNA in a mother's blood. It's thought that the test can detect 99% of all Down syndrome pregnancies. It also tests for some other chromosomal abnormalities.

Ultrasounds: Ultrasounds (sonograms) are commonly offered week 20, although they may be done at any time during a pregnancy. A sonogram may be offered for a variety of reasons, including verifying a due date, checking for multiple fetuses, investigating complications such as placenta previa (a low-lying placenta) or slow fetal growth, or detecting malformations like cleft palate. Sometimes, if there are findings suspicious for a genetic abnormality seen, you may be referred to a specialist or for further genetic testing. During the procedure, a device is moved across the abdomen that transmits sound waves to create an image of the uterus and fetus on a computer monitor. New three-dimensional sonograms provide an even clearer picture of your baby, but they aren't available everywhere and it's not clear whether they're any better than two-dimensional pictures in contributing to a healthy pregnancy or birth.

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Glucose screening: Typically done at 24 to 28 weeks, this is a routine test for pregnancy-induced diabetes, which can result in overly large babies, difficult deliveries, and health problems for you and your baby. This test measures your blood-sugar level an hour after you've had a glass of a special soda. If the reading is high, you'll take a more sensitive glucose-tolerance test, in which you drink a glucose solution on an empty stomach and have your blood drawn every hour for 2-3 hours.

Amniocentesis: This optional test is usually performed between 15 and 18 weeks of pregnancy (but can be done earlier) for women who are 35 or older, have a higher-than-usual risk of genetic disorders, or whose MSAFP, multiple marker, or cell-free DNA test results were suspicious. The procedure is done by inserting a needle through the abdomen into the amniotic sac and withdrawing fluid that contains fetal cells. Analysis can detect neural tube defects and genetic disorders. The miscarriage rate after amniocentesis varies depending on the experience of the doctor performing the procedure, averaging about 0.2% to 0.5% at 15 weeks, but the test can detect 99% of neural tube defects and almost 100% of certain genetic abnormalities.

Fetal Doppler ultrasound: A Doppler ultrasound test uses sound waves to evaluate blood as it flows through a blood vessel. Fetal Doppler ultrasound can determine if blood flow to the placenta and fetus is normal.

Fetoscopy: Fetoscopy allows a doctor to see the fetus through a thin, flexible instrument called a fetoscope. Fetoscopy can detect some diseases or defects that cannot be found by other tests, such as amniocentesis, ultrasound, or chorionic villus sampling. Because fetoscopy carries significant risks for the mother and fetus, it is an uncommon procedure that is only recommended if there is a much higher-than-normal chance that the baby will have an abnormality.

WebMD Medical Reference Reviewed by Traci C. Johnson, MD on February 03, 2017

Sources

SOURCES:

Gabbe: Obstetrics-Normal and Problem Pregnancies, 4th edition, 2002.

WebMD Medical Reference provided in collaboration with The Cleveland Clinic: "Pregnancy: Your Guide to Pregnancy."

WebMD Medical Reference from Healthwise: "Rh Sensitization During Pregnancy;" "Doppler Ultrasound;" and "Fetoscopy."

© 2017 WebMD, LLC. All rights reserved.

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