Its the rush of blood to the cervix that causes labor. When you you have your O the cervix fills with blood and your uterus muscles start to cramp which triggers contractions. My doctor told me that you experience minor contractions that you may not know are contractions once you are closer to your due date.....this is preparing the cervix and uterus for the birth. This is why when you have sex closer to your due date you may go onto labor....your body is already preparing for labor and an O can jump start it!
Sex can start labor a number of ways the first is the roughness of the penis near your cervix which can cause contractions. second, sperm has a chemical that affects the cervix and causes dilation. third different positions can can irriation on the cervix and cause you to contract and dilate.
also if you orgasim, thats your uterus contracting and it can keep on contracting so you start labour, it can also trigger the release of oxytocin the "contraction"hormone and semen contains a high concentration of prostaglandins which may help to ripen/soften the cervix :)
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