Many men are concerned about hurting their pregnant partner, or their baby, when having intercourse during pregnancy. Penetrative sex won't hurt your unborn baby. The cervix is sealed with a thick mucous plug that helps prevent infection and the bag of waters (membrane) and amniotic fluid, act as a cushion to protect your baby. However, a frequently asked question is "Will orgasm for the woman start labour?" The answer to this is "No", unless her body is already prepared to start labouring, near the end of the pregnancy.
The uterus (or womb) is an organ that contracts regularly from the time of a girl's first period to menopause (when she stops menstruating). During the last months of pregnancy the uterus is quite active with Braxton Hicks contractions. These are the practice contractions, the 'dress rehearsals' for labour. When a pregnant woman has an orgasm, she will usually experience some Braxton Hicks contractions for a few minutes afterwards. This is normal and will not start the labour. (Sex is now not believed to be a cause for
premature labour. If your partner starts labouring prematurely, her body was preparing for this anyway. It was not because you had sex recently). Towards the end of the pregnancy, as her body prepares to go into labour, prostaglandins in your semen and her experiencing an orgasm are capable of triggering and stimulating labour contractions to start, but a woman's body must be 'ready' to react in this way.
During the middle phase of pregnancy many women find they have a renewed interest in sex, often because they feel physically better and more energetic. Her pregnant belly is not very big yet, making her feel relatively comfortable (compared to later in the pregnancy). You can read more in
sex middle months of pregnancy.
Be aware of your own feelings about sex during pregnancy. Many partners feel uneasy about having 'another person' between them and their partner.