I am not sure what type of complications you've had, but a large number of twin pregnancies are high risk. I would say it would not be worth the risk. The reproductive organs sit in close proximity to the lower bowel, and since you are over halfway through your pregnancy your enlarged uterus is subject to trauma from anal sex..which may in turn stimulate contractions or bleeding.
Although not satisfying to you I would suggest other things to stimulate and satisfy your partner.
While this subject seems quite embarrasing to discuss with your Dr. I think it's a very good question. Best wishes to you and your babies. [ Nallie's advice column | Ask Nallie A Question ]
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