penicillin is the drug of choice for strep so maybe there was a group b streptococcal infection discovered prenatally that needed treatment. this is relatively common. this is just a guess however. your son and his gf can certainly answer that question if they want to you to know.
my son was given penicillin after his gf had prenatal ck up. gf had discharge something in iv dur. labor. what for? . she asked me several times while she was pregnant if it was normal to have lots of discharge clumps of it i thought it may have been her mucous plug but upon an exam a few days after asking me her plug was still intact. she said she had the discharge for a while and seemed to leek a lot. like running down her legs leaving wet spots on bed ect. then she tells me that they have to put something in her iv when she goes into labor to prevent the baby from being infected claims she doesn't know