Third Trimester (from 28 weeks to delivery, around 40 weeks)
The laboratory tests used during these weeks are primarily directed toward preparing for the birth of a healthy baby and may include the following:
- Urine screen for sugar and/or protein
- Urine culture to detect bacteriuria (may be repeated)
- Group B streptococcus
- Hemoglobin and platelet count
Other less routine tests include:
- Antibody screen
- Fetal fibronectin
- Amniocentesis – to assess lung development for babies at risk of premature delivery
If a woman participates in high-risk activities that may transmit HIV or hepatitis B, such as unprotected sexual contact or intravenous drug use, retesting for these infections during the third trimester is recommended. It is also recommended, and in some states required, that testing for sexually transmitted diseases like gonorrhea, chlamydia, and syphilis be performed or repeated in the third trimester. (See the Pre-conception section for additional information.)
The pregnant woman may also be given a non-stress test (NST). This non-invasive test, given after week 28, is used mostly in high-risk pregnancies or when a baby is past due to monitor its health and look for signs of distress. The test measures the fetus' heart rate as it relates to movement.
When the health care provider is concerned about how contractions will affect the fetus' heart rate, the non-stress test may be followed by a contraction stress test. The woman is given a medication that induces mild contractions (stress), after which the heart rate of the fetus is again monitored.