The next pregnancy the mother will completely reject the baby's blood if this is Rh-positive.
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Of course there are some exceptions such as the isoimmunization of baby's red blood cells (RBCs)(say, Rhesus factor positive) in a mother who is Rh-negative during the first pregancy. Why doesn't the woman reject such implant? The fact is that many (or most) pregnancies are carried to term without rejection. This is like an implant, a transplant, a graft. During pregnancy, the woman has to nurture a baby who has 50% of the genes coming from another person. Besides that, semen could potentially immunize the woman against her partner. Semen can carry many pathogens and thus it's perfect to transmit any STD if present. To think that it will always stay inside somehow is not realistic. And finally, semen deposited in the rectum will come out.with bowl movements, setting on the toilet, etc. The risks of his ejaculating inside you are similar with either vaginal or anal sex in this regard. However, if this is a casual encounter, and you are not sure of the sexual history of your partner, you'll want to use a condom during ANY sexual encounter. Onto another aspect: If you are in a longtime committed relationship, exclusive partners may choose not to wear a condom and ejaculate into the anus. But with any reasonable care, cleaning at the toilet, being careful not to lay in a position where "dripping" across the vagina is likely, you should be alright. However, if semen or sperm leak out and drip across the vulva or vaginal opening, there is a risk, however small, of pregnancy.
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First, the pregnancy risk with anal sex only is virtually nil. The above feedback is very good, but needs to be fine tuned. As far as the safety of ejaculating during anal sex, if you feel no discomfort tell him to go deep and fire torpedoes, it's completely safe. The only way this is possible is if semen leaks out of the anus and comes in contact with the vagina. As for pregnancy, it is not unheard of for a woman to become pregnant from anal sex, but it is highly unlikely. Gravity is also a factor in how long it takes for semen to pass from the rectum, as if you immediately get up and stand around it will obviously come out faster than if you immediately roll over and go to sleep.
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When you do either, your partner's semen will ooze out. However, once you are relaxed it will need to be expelled and either start to leak out of the anus as it does the vagina, or more commonly, you will feel as if you need to either use the bathroom or pass gas. Ejaculate will likely not come out of the anus immediately following anal sex due to the anal sphinctors as previous posters have pointed out. The semen will come out, but it depends on two scientific factors known as gravity and relaxation.