Luis,
After she is 37 weeks gestation, it is very safe to have sex with her. The semen contains prostaglandin which causes the pregnant woman to have contractions. It is safe for her to go in labor after 37 weeks since the baby's lungs are already mature in case she delivers. I have seen patients who come to Labor and Delivery complaining of labor pains at 28 weeks gestation because they had sex with their partners. Sometimes they get admitted to the hospital for Preterm Labor.
When you need to approach the one you love in this condition, patience and tenderness have to be given, many men still think that there climax is what they need, no way, the person you share your life with deserves so much more, especially when in her condition, with the right procedure , you both can enjoy sex till both come to the point for her saftey. Enjoy, but respect.
Sex should be fine unless OBGYN says different. Remember , love and tenderness.
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