She obviously cheated on you with a vampire because everyone knows you get your period when your pregnant...with human babies ;-P

Edit: Okay if you're not joking then look it up. The pregnancy hormone the woman's body secretes upon conception prevents the lining of the uterus from shedding. The pregnant woman will then not get her period. The baby lives in amniotic fluid and at six months the baby is drinking that for 'practice'. All nutirents are provided through the placenta and the umbilical cord. Google this!