My girlfreind’s aunt just had a baby, and she told my girlfreind that she didn’t even know she was pregnant until about the 4th month, because she was still having her period on a regular basis. Now this has got my girlfreind all worried, and I was just looking for information on how common this actually is. If possible, could you provide a cite? Thanks
There is anecdotal evidence for periods occurring during pregnancy, but they tend to be FOAF stories.
I would guess that a genuine period would be fatal to a pregnancy, but that some spotting might occur. If a woman had irregular periods or inconsistent flows during her periods, she might conclude that the spotting was a “period.”
Bleeding During Pregnancy from Mt. Auburn OB/GYN.
Again, I can only offer anecdotal evidence here, but from what my friends have told me and also what my obstetricians have, some women do continue to have what can be interpreted as “periods” during the first few months. My obstetricians have also told me that these bleeds are usually somewhat different in nature than a “normal” period - often much lighter and of less duration. Some women “spot” throughout most of their pregnancies. As far as I understand the phenomenon though, having continued “normal” (for the woman concerned) periods througout pregnancy is an extremely rare phenomenon.
My mother didn’t know she was pregnant with me for the first few months, because she had periods (albeit lighter).
The pregnancy was fine, I was a healthy birthweight, the only thing my mother hasn’t forgiven me for was giving her a 36-hour labour in the days before epidurals were available.
So that’s the best cite I can give you.
A few developmental variations in the normal uterus can lead to normal menstrual bleeding during pregnancy. For example, the uterus and vagina ach develop in two halves (left and right) which fuse to form one common chamber. In some women, the fusion may not take place or may only be partly complete - there are numerous variations, but in the most extreme case of a double uterus a fetus in one uterus does not prevent bleeding from the other uterus. A similar condition can occur with a bicornuate or partly fused uterus, where one “horn” can still shed it’s lining.
Kevin Luong
Well, I’m pregnant, and I thought I got my period. It was just spotting. My periods are very variable, and the spotting occured about when I was supposed to get my period, so I figured it was just a (very) light period.
I went to the doctor a few weeks later because I felt so incredibly lousy. Boy was I surprised to find out that I was almost 2 months pregnant! 
The only person I know IRL who went through this is a family member. She was scheduled to get a hysterectomy, and had to take a pregnancy test beforehand. Her doctor assured her this was just a formality; she was old enough, and having enough medical issues, that everyone assumed they knew the result ahead of time. (This was more than 30 years ago; I’ve been told that abortion laws in that time and place required a pregnancy test on all women undergoing a hysterectomy, even those who couldn’t possibly be pregnant.) She, her family and her doctor were all jaw-droppingly surprised when it came back positive. She’d just finished what she thought was her last menstrual period.
Note, though, that she was having “female problems” (no one has ever seen fit to tell me what) which is why she was scheduled for surgery in the first place. So this doesn’t count as a healthy woman having periods while pregnant. The pregnancy resulted in a healthy baby boy, and the new mom had the hysterectomy some months later than planned…
This seems in line with what you read about the subject in Cosmo; take that for what it’s worth.
KevinL, welcome to the SDMB and thanks for the interesting post.
–Cliffy
Suppose two eggs are released. The common story is that they both get fertilized and you have fraternal twins. But, suppose one egg doesn’t get fertilized? (Maybe highly improbable, but a possibility I’d assume.) Could the body be tricked into having a period while pregnant?
It just doesn’t seem possible considering what must physically happen to promote the growth of a fertilized egg. Maybe the second egg just dies and may pass as “spotting” or as part of some small mucousy discharge?
I have to admit it is very hard for me to buy these stories of women who claim that they didn’t know they were pregnant far too many months into the whole deal. - Jinx
Congrats, Green-Bean!!!
There is also the possibility of a first “period” after pregnancy being implantation blood. After the fertalized egg nestles itself in the warm and cozy lining of the uterus, some of the rest of the lining can be shed. It usually looks like an unusual period, about a few days later than expected, darker and thicker (not heavier).
Women who get this often “determine” that they are not pregnant but shortly there after start to feel the effects of the pregnancy.