A group of doctors in Canada have published a set of guidelines in which they say that sex during low risk pregnancy is completely safe

Lead researcher Dr Clair Jones said that having sex during pregnancy is completely normal. “Sex in pregnancy is normal. There are very few proven contraindications and risks to intercourse in low-risk pregnancies. And therefore these patients should be reassured”, Dr Jones said.
However adequate precaution should be taken in high risk pregnancies with doctors suggesting that it is safer to avoid sex until the pregnancy is complete. Some of the cases in which sex should be avoided include when there is a history of early labor or if the woman is suffering from genital tract infection or a condition called placenta previa.
Source-Medindia