Every mother is supposed to have a blood test for syphilis at or shortly before the time of delivery. The test detects the presence of antibodies to syphilis and the result is expressed as a a titer, e.g. 1:1, 1:2, 1:4, and so on. The higher the titer, the easier it is to detect the antibodies. A problem is that after someone has syphilis, the test for it can remain positive for years, making it difficult to know for sure whether a positive test is due to new infection or to old, previously treated infection. Sometimes by history we can sort it out, but other times we simply cannot. Since untreated syphilis can be transmitted to the baby in utero and lead to bad problems such as insanity or death, we err on the side of treatment if there is uncertainty of the diagnosis.
We currently are treating a baby for possible syphilis whose mother had syphilis in 1997 that was, as far as we can tell, treated adequately. Her syphilis titer in this pregnancy went from 1:2 originally to 1:4 and then 1:8 at the end of the pregnancy. Is this just some variation in the lab test, or new infection in the mother? The baby's titer is also 1:8. Is that just transferred old antibody from the mother, or is it real infection in the baby? We aren't sure, but the chances of new infection in the mother - and passing it on to the baby - are great enough that we did a spinal tap on the baby to rule out nervous system syphilis and are treating the baby with ten days of penicillin.
It can be a little tricky sometimes talking to parents about syphilis. Mom's usually embarassed, and you have to try to make them understand the antibody thing, then convince them to let you do a spinal tap on their seemingly healthy baby and keep the baby in the hospital for ten days of treatment. But I bet it's trickier for the mother to discuss it with her partner and other relatives. She almost has to either accuse her partner or herself of infidelity - no small thing - or else claim it's just laboratory variation in the antibody testing, which is pretty hard for one layperson to explain to another.
For crying our loud, don't sleep around, and if you do, use protection. Your baby might thank you for it someday.