sex should be perfectly safe for her unless specified by her doctor. i had bleeding problems with my first baby Sawyer, so i wasnt allowed to exercise or have sex. generally her birth canal is about 9 inches long where as the average penis is only around 6 inches, plus there is also the cervix and the birth sack membranes that protect the child. the final line of defence is the amniotic fluid to cushon the baby. many pregnant women have fallen flat on their stomachs and saught medical help out of fear after and nothing was wrong with the baby.
now, be aware that your ejaculate contains hormones which can cause the cervix to ripen (generally only when her body is ready) and orgasm can induce uterine contractions. many people have used sex to induce labour in many cultures, i would however use a condom. i dont want to scare anyone or gross them out, but with my first son Sawyer, we used unprotected sex to induce labour which worked like a charm (i dialated 4 cm right after our first try, which stalled. on our second try my water broke and i gave birth the next day). the problem was that due to the penetration, bacteria was introduced into my birth canal. my son got stuck in my pelvis and we needed to use forceps to pull him out. when you have a bruise your pores dialate and the doctors think that this was how bacteria got into Sawyers blood stream. we never knew until it was too late... he passed away from meningitis at 12 days old and it was so preventable.