Why early screening matters
Many sexually transmitted infections cause no symptoms, so a woman may not know she is infected. Chlamydia, for example, is common in women and men without any symptoms, and screening is the main way it is detected. An untreated infection in pregnancy can harm the baby. Syphilis can pass to the fetus and cause miscarriage, stillbirth, premature birth, low birth weight or death soon after birth. Some babies with congenital syphilis appear well at birth and develop serious problems later. Syphilis is curable, and prompt treatment of the mother and her sexual partners helps prevent these outcomes.
Tests at the first antenatal visit
Guidance from the United States Centers for Disease Control and Prevention (CDC) recommends that all pregnant women are tested for syphilis, HIV and hepatitis B at the first antenatal visit. Hepatitis B testing is repeated in every pregnancy, regardless of earlier results. Hepatitis C screening is recommended in pregnancy except in settings where the infection is very rare. Chlamydia and gonorrhoea screening is recommended for women under 25 years and for older women at increased risk.
Some tests are repeated later in pregnancy. Women at increased risk of syphilis are retested at 28 weeks and at delivery, women at high risk of HIV are retested in the third trimester, and women under 25 or at risk of chlamydia or gonorrhoea are retested in the third trimester.
Chlamydia and the newborn
Chlamydia can pass to the baby during birth from an infected cervix. In newborns it most often causes conjunctivitis 5 to 12 days after birth, or pneumonia at 1 to 3 months of age. Screening and treatment during pregnancy are the best way to prevent this. A pregnant woman treated for chlamydia should have a test of cure about four weeks after treatment and be retested about three months later.
Herpes and other organisms
CDC guidance does not recommend routine blood testing for herpes simplex virus type 2 in pregnant women without symptoms, although type-specific tests can help in some situations, such as counselling about the risk of acquiring genital herpes during pregnancy. Trichomoniasis is not on the CDC list of routine screening tests for all pregnant women. Testing for these and other organisms is decided by the treating clinician according to symptoms and risk.
Group B Streptococcus late in pregnancy
Group B Streptococcus (GBS) is a bacterium that many healthy women carry without symptoms, but it can pass to the baby during birth. CDC advises screening for GBS during the 36th or 37th week of each pregnancy. A clinician uses a sterile swab to collect a sample from the vagina and rectum. Women who test positive are given antibiotics during labour to help protect the newborn from infection.
Follow local antenatal guidance
Screening schedules differ between countries and health services, and the recommendations above are from United States guidance. Pregnant women should follow the antenatal testing plan advised by their own doctor or midwife and ask about any result they do not understand. This guide is general information and is not medical advice.