Check out this startling photo gallery of celebrity cheating while wife is pregnant at Slate.com (click on the individual pictures for the backstory). Did you realize these guys had pregnant wives at home while they were roaming - some getting other women pregnant at the same time as their wives were carrying their babies? I must have missed that part, or maybe it isn't played up much in the reporting that gets done.
There's plenty of speculation, personal experiences, and even a pretty good article here and there on the web on cheating and pregnancy, but nobody seems to have any statistics to tell us how frequently this happens. Is it 10% of the time, 20%, 50%, inevitable?
There is agreement that pregnancy doesn't make a man a cheater. While "she's too fat to be attractive" or "she doesn't feel like having sex" might be heard from guys as excuses as to why cheat on a pregnant spouse, the consensus is that pregnancy doesn't turn a faithful guy into a cheater, and guys who cheat ultimately need no excuse, or rather, any excuse will do.
It does appear to be true that becoming pregnant, either by your boyfriend or your husband, makes it more likely that you will experience intimate partner violence (domestic abuse).
The Center for Disease Control defines domestic violence during pregnancy as “Physical, sexual, or psychological/emotional violence, or threats of physical or sexual violencethat are inflicted on a pregnant woman.” In a household survey, it was found that pregnant women are 60.6% more likely to be beaten than women who are not pregnant. Violence is cited as a pregnancy complication more often than diabetes, hypertension or any other serious complication. (“Battering and Pregnancy” Midwifery Today19: 1998). [World Health Organization Fact Sheet]Emergency rooms know about pregnancy abuse, police know about it, DV hotlines, Departments of Family Services, and social workers have it on their radar. The ones who are in the dark, and usually taken completely by surprise, are the women themselves. Having a baby with a man you love makes you a family, is supposed to create a wonderful bond between the two of you, and if there are troubles in the relationship, conventional wisdom says there's nothing like a new baby to change the dynamic. The little one is half you/half him so loving your baby can't help but make you closer. Makes so much sense, but apparently not real life.
If you'd like to know more about abuse during pregnancy, here's an excellent article, complete with a hotline number to get more information and help. The World Health Organization Fact Sheet cited above is also excellent although currently inaccessible for some reason. Both articles have a downside is that they focus on physical abuse, and say almost nothing about financial abuse, jealousy, isolation, psychological harassment, stalking or other demeaning behaviors (I would put cheating in this category).
If only women could know in advance (maybe a blood test?) who those cheaters are, that would be the greatest leap forward for pairing up and starting a family since eHarmony's "Scientific Relationship Questionnaire came online. I can't imagine that any of the women pictured in the Slate.com photo gallery of cheaters married their guy thinking he would behave as badly as he did, even while they were pregnant with his children.