How STDs Affect Pregnancy: Risks, Testing, and What to Do

STDs can affect pregnancy in ways that neither the mother nor her care team can predict without testing — because most of the infections that cause the most harm are entirely asymptomatic. A pregnant woman with chlamydia, gonorrhea, or syphilis may have no symptoms at any point during the pregnancy while the infection causes complications that range from preterm birth to stillbirth to severe neonatal illness.

This is why prenatal STD screening is a clinical standard, not an optional extra. The infections are detectable. The consequences of missing them are serious and largely preventable.

Quick answer: The CDC recommends all pregnant women be tested for syphilis, HIV, hepatitis B, and chlamydia at the first prenatal visit. High-risk patients should be retested in the third trimester. Gonorrhea testing is recommended at the first visit for women under 25 or with risk factors. Untreated syphilis is the most acutely dangerous — it can cause stillbirth and is entirely preventable with penicillin. If you are pregnant and haven’t been tested, same-day testing is available in Los Angeles, Houston, New York City, Miami, and Chicago.

Which STDs Are Most Dangerous in Pregnancy

Syphilis

Congenital syphilis is the most preventable catastrophic STD outcome in pregnancy, and one of the most alarming — US rates have increased by over 750% since 2012. An infected mother who is not identified and treated passes the spirochete to the fetus through the placenta. The consequences depend on the stage of syphilis and the timing of treatment: early in pregnancy, untreated syphilis causes miscarriage, stillbirth, and severe fetal malformations. Later, it causes neonatal infection that affects the bones, liver, nervous system, and skin of the newborn. Penicillin during pregnancy is highly effective and prevents congenital syphilis when administered in time. There is no reason any baby should be born with syphilis — every case represents a gap in prenatal care.

HIV

Without treatment, HIV transmission from mother to child during pregnancy, delivery, or breastfeeding occurs in 15–45% of cases. With effective antiretroviral therapy during pregnancy, this rate drops to below 1%. A pregnant woman with HIV who maintains an undetectable viral load on treatment has a very low probability of transmitting the virus to her baby. This is one of the most effective interventions in all of medicine — which is why HIV testing at the first prenatal visit is essential. Late diagnosis, or missing the diagnosis entirely, forecloses these protections.

Gonorrhea

Gonorrhea can cause chorioamnionitis — infection of the membranes surrounding the fetus — which leads to preterm labor, premature rupture of membranes, and low birth weight. During delivery, the bacteria can infect the newborn's eyes (ophthalmia neonatorum), causing blindness if untreated. Neonatal eye drops (erythromycin or silver nitrate) are standard practice at birth in the US, but identifying and treating the mother before delivery is the more effective intervention. Gonorrhea during pregnancy also increases the risk of disseminated gonococcal infection in the mother — a systemic, potentially life-threatening condition.

Chlamydia

Chlamydia is the most common bacterial STD in the US and is predominantly asymptomatic, including in pregnant women. Untreated chlamydia in pregnancy increases the risk of preterm birth, premature rupture of membranes, and low birth weight. During vaginal delivery, chlamydia can be transmitted to the newborn, causing neonatal conjunctivitis and, more seriously, neonatal pneumonia — which typically presents at 1–3 months of age and can be severe. Treatment with azithromycin or amoxicillin during pregnancy is safe and effective.

Herpes (HSV)

Neonatal herpes is rare but serious. It typically occurs when a mother has a primary (first) genital herpes infection in the third trimester and has not yet developed antibodies — meaning the virus passes to the baby during delivery with no maternal immune protection. Recurrent herpes in women with long-established infection carries much lower neonatal risk because maternal antibodies provide some protection. Suppressive antiviral therapy from 36 weeks of pregnancy reduces the risk of herpes outbreaks at delivery and is recommended for all women with a history of genital herpes. Caesarean delivery is indicated when there is an active genital herpes outbreak at the time of labor.

Hepatitis B

Hepatitis B can be transmitted from mother to newborn during delivery. Without intervention, babies born to hepatitis B-positive mothers have a 40–90% chance of developing chronic hepatitis B infection, which carries lifetime risk of cirrhosis and liver cancer. Vaccination of the newborn within 12 hours of birth, combined with hepatitis B immune globulin (HBIG), prevents transmission in over 95% of cases. This intervention only works if the mother's hepatitis B status is known — which requires first-trimester testing.

CDC Prenatal STD Screening Recommendations

All pregnant women at the first prenatal visit: syphilis (RPR or VDRL), HIV (4th generation test), hepatitis B surface antigen, and chlamydia NAAT. Gonorrhea NAAT at the first visit for women under 25 or with risk factors (new partner, multiple partners, inconsistent condom use, previous STD). Hepatitis C testing for women at risk (injection drug use, blood transfusion before 1992, HIV-positive). Repeat syphilis, gonorrhea, and HIV testing in the third trimester for high-risk women, ideally before 36 weeks. Herpes cultures at the onset of labor for women with known HSV. GBS (group B streptococcus) screening between 36–37 weeks — this is not an STD but is part of standard prenatal infection screening.

If You Test Positive During Pregnancy

A positive result during pregnancy requires prompt response, but it is not a catastrophe. The outcomes that make STDs dangerous in pregnancy are the ones that come from late or missed detection. Early detection gives you and your OB time to treat effectively before the risk window.

Syphilis: penicillin G. Safe in pregnancy at all stages. The regimen depends on the stage of syphilis. Treatment earlier in pregnancy produces better outcomes. Retest to confirm serological response 1 month after treatment and again at delivery.

HIV: antiretroviral therapy should be started as early as possible if not already underway. Goal is an undetectable viral load by the third trimester. Delivery planning — caesarean vs. vaginal — is guided by viral load at 36 weeks.

Chlamydia: azithromycin (1g single dose) or amoxicillin. Both are safe in pregnancy. Doxycycline is the standard non-pregnant treatment but is not recommended in pregnancy. Test of cure at 3–4 weeks after treatment and retest at 3 months.

Gonorrhea: ceftriaxone (single IM injection). Safe in pregnancy. Azithromycin is no longer routinely added due to resistance concerns. Test of cure recommended due to antibiotic resistance.

Hepatitis B: newborn vaccination and HBIG within 12 hours of birth. For mothers with high viral loads, tenofovir in the third trimester reduces perinatal transmission further.

When to Seek Urgent Care

• Positive syphilis result at any point in pregnancy: contact your OB the same day. Penicillin treatment should begin as quickly as possible.

• Positive HIV result in a pregnant woman not on treatment: same-day referral to infectious disease or maternal-fetal medicine is appropriate.

• Painful genital sores in late pregnancy: same-day evaluation to rule out active herpes and plan delivery mode.

• Fever, chills, or pelvic pain during pregnancy: same-day or ER evaluation — possible chorioamnionitis or PID requiring IV antibiotics.

Frequently Asked Questions

Is it safe to treat STDs during pregnancy?

Yes — all of the major bacterial STDs (chlamydia, gonorrhea, syphilis) are treatable with antibiotics that are safe in pregnancy. HIV is managed with antiretrovirals that are safe and effective in pregnancy. The risk of not treating is far greater than any theoretical risk from treatment. Doxycycline (the standard chlamydia treatment) is the main exception — it is not used in pregnancy, but safe alternatives exist.

I tested negative in the first trimester. Do I need to test again?

For high-risk women, yes — the CDC recommends retesting for syphilis, gonorrhea, and HIV in the third trimester, ideally before 36 weeks. If your risk level has not changed since early pregnancy, one test is generally sufficient for most infections. Discuss your specific situation with your OB.

Can I have a vaginal birth if I have an STD?

It depends on the infection. Active genital herpes at the time of labor is a reason for caesarean delivery. For most other infections (chlamydia, gonorrhea, syphilis, HIV with undetectable viral load), vaginal delivery is not contraindicated once treatment has been completed or is underway. Your OB will make a delivery plan based on your specific situation.

My partner tested positive but I tested negative. What should I do?

Disclose the positive result to your OB immediately. If your partner has chlamydia, gonorrhea, or syphilis, you may have been recently exposed and could test negative within the window period. Your OB may recommend treating you as well, or repeating the test at the appropriate window. Partner testing and treatment simultaneously with your own treatment prevents reinfection.

Does having an STD during pregnancy mean my baby will be infected?

Not necessarily — and in most cases, with prompt treatment, no. The risk of transmission depends on the infection and the timing and completeness of treatment. For syphilis, treatment with penicillin before 26 weeks of pregnancy is highly effective at preventing fetal infection. For HIV, treatment resulting in an undetectable viral load reduces transmission to under 1%. The goal of prenatal STD screening is precisely to enable these interventions before transmission occurs.

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