Syphilis in Pregnancy and the Baby's Risk

What is congenital syphilis, and how does it happen?

Congenital syphilis happens when a pregnant person with untreated syphilis passes the infection across the placenta to the fetus, at any point in pregnancy and at any stage of their own infection 1Ref 1Centers for Disease Control and Prevention (2024).About Congenital Syphilis.Supports that syphilis crosses the placenta and causes congenital syphilis, that untreated infection can cause stillbirth, infant death, and severe newborn outcomes, and that penicillin during pregnancy is the only treatment proven to prevent these outcomes.. The bacterium crosses freely into the shared bloodstream once someone is infected, so the exposure happens in utero, not during delivery the way some infections are passed from parent to baby.

That timing matters for two reasons. A cesarean delivery does not prevent transmission, because the infection has often already reached the fetus long before labor starts. And the earlier in pregnancy the parent is treated, the more time the fetus has to be protected — which is the entire argument for testing early rather than waiting for symptoms that, in this infection, usually never show up.

Why does prenatal screening matter more than symptoms?

Prenatal care includes a syphilis test because the infection is often silent in a pregnant person's own body, and waiting for symptoms to prompt testing misses the window where treatment still protects the fetus. Screening pregnant patients for syphilis is part of the standard recommendation for STI testing during pregnancy 2Ref 2Centers for Disease Control and Prevention (2021).STI Screening Recommendations.Supports that pregnancy screening for syphilis is part of CDC's population-specific STI screening recommendations, used here to explain why prenatal care includes a syphilis test regardless of symptoms., precisely because relying on how someone feels is not a reliable way to catch it.

Syphilis moves through stages, and the middle ones can pass without anyone noticing. An initial sore is often painless and can appear somewhere it is never seen; a later rash can come and go on its own; after that, the infection can sit in latent syphilis, a stage with no symptoms at all, for months or years 3Ref 3Centers for Disease Control and Prevention (2024).About Syphilis.Supports the staged natural history of syphilis — a painless primary sore, a secondary rash, and a symptomless latent stage — used here to explain why the infection is easy to miss during pregnancy.. Because syphilis symptom mimicry is so well documented — the rash in particular can resemble dozens of other conditions — pregnancy is exactly the setting where relying on appearance rather than a blood test is riskiest.

Syphilis is one of several stis in pregnancy that a standard prenatal panel screens for; chlamydia in pregnancy and trichomoniasis in pregnancy are two others covered elsewhere, each with its own risk profile for a pregnancy.

What untreated syphilis can do to a pregnancy

Untreated syphilis in pregnancy can cause miscarriage, stillbirth, or a baby's death shortly after birth, and it is one of the few infections where the outcome can be that severe even when the pregnant person feels completely well 1Ref 1Centers for Disease Control and Prevention (2024).About Congenital Syphilis.Supports that syphilis crosses the placenta and causes congenital syphilis, that untreated infection can cause stillbirth, infant death, and severe newborn outcomes, and that penicillin during pregnancy is the only treatment proven to prevent these outcomes.. Babies who survive can be born with serious, sometimes lifelong, effects: bone and joint problems, anemia, an enlarged liver or spleen, jaundice, and, in some cases, damage that only becomes apparent later, including hearing loss or bone deformities that surface after infancy.

Penicillin is the only treatment shown to both cure the pregnant person's infection and prevent these outcomes in the fetus 1Ref 1Centers for Disease Control and Prevention (2024).About Congenital Syphilis.Supports that syphilis crosses the placenta and causes congenital syphilis, that untreated infection can cause stillbirth, infant death, and severe newborn outcomes, and that penicillin during pregnancy is the only treatment proven to prevent these outcomes.. That single fact is why prenatal screening exists in its current form: it is not a general precaution, it is the mechanism that catches a treatable infection before it does damage that cannot be undone.

How is congenital syphilis treated during pregnancy?

Penicillin is the only treatment proven both to cure a pregnant person's syphilis infection and to prevent it from harming the fetus 1Ref 1Centers for Disease Control and Prevention (2024).About Congenital Syphilis.Supports that syphilis crosses the placenta and causes congenital syphilis, that untreated infection can cause stillbirth, infant death, and severe newborn outcomes, and that penicillin during pregnancy is the only treatment proven to prevent these outcomes.. No other antibiotic has been shown to reliably treat an infected fetus the way penicillin does, which is why guidelines do not offer alternatives as equivalent options during pregnancy.

Started early enough in pregnancy, treatment is highly effective, and many babies are born with no signs of infection at all. Timing is what determines how protective it is — treatment in the first half of pregnancy gives the fetus the most protection, while treatment very close to delivery may not leave enough time to prevent every effect. Anyone with a documented penicillin allergy is not simply switched to a different antibiotic for this infection in pregnancy — penicillin is treated as the only proven option, which is part of why a prenatal visit specifically asks about drug allergies rather than treating it as a minor detail.

What happens after the baby is born?

A baby born to a parent who had syphilis during pregnancy is evaluated at birth, because no test can fully confirm in advance that a fetus was unaffected. Some effects of congenital syphilis do not show up immediately — certain bone and hearing problems can surface months or years later 1Ref 1Centers for Disease Control and Prevention (2024).About Congenital Syphilis.Supports that syphilis crosses the placenta and causes congenital syphilis, that untreated infection can cause stillbirth, infant death, and severe newborn outcomes, and that penicillin during pregnancy is the only treatment proven to prevent these outcomes. — which is why follow-up continues even when a newborn looks completely well at first.

A normal first exam is reassuring but not the end of the process. Because the timing of the parent's treatment during pregnancy affects how much protection the fetus received, a baby's follow-up plan is built around when in the pregnancy treatment happened, not just whether it happened at all.

Can congenital syphilis be prevented?

Congenital syphilis is preventable, and the prevention is almost entirely about catching and treating the parent's infection early rather than protecting the fetus directly. Condoms reduce the risk of catching syphilis during sex, but they only cover the skin they cover — syphilis spreads through skin-to-skin contact with a sore, so a condom offers only partial protection rather than the near-complete protection it gives against infections spread through fluids alone 4Ref 4Centers for Disease Control and Prevention (2024).Condom Use: An Overview.Supports that condoms offer only partial protection against infections spread by skin-to-skin contact, such as syphilis, compared to the near-complete protection they offer against fluid-borne infections..

The most protective action during pregnancy is not a barrier method — it is completing prenatal syphilis testing and getting treated immediately if it comes back positive. Understanding why syphilis is surging again matters here too: adult infections have been climbing in the United States for years, and while 2023 was the first year primary and secondary cases fell in more than two decades, more than 209,000 cases were still reported that year 5Ref 5Centers for Disease Control and Prevention (2025).National Overview of STIs in 2023.Supports 2023 U.S. syphilis surveillance figures — over 209,000 reported cases, with primary and secondary cases falling for the first time in over two decades — used here to give scale to the adult infection rates that drive congenital syphilis risk. — a scale of adult infection that keeps congenital cases in the picture. A sexual partner also needs to be tested and treated, since re-exposure during the same pregnancy can undo the protection treatment already provided.