We advise couples not to be sexually active immediately after IVF

Dr Simon Fishel

Lead researcher Kelton Tremellen found a 50% improvement in the number of embryos that successfully implanted.

However, most of these involved extra multiple pregnancies, such as twins and triplets, rather than more singleton pregnancies.

The researchers suggest that a component of semen may have the effect of affecting the immune system so that it does not reject the new embryo.

Genetic defects

Although two or sometimes three embryos are implanted in the womb, many do not develop into pregnancies, and others miscarry at a very early stage, although scientists are not entirely sure why.

It has been suggested that some embryos carry genetic defects which mean they can never become a child.

Dr Simon Fishel, Director of the Centres for Assisted Reproduction at the Park Hospital in Nottingham, told BBC News Online that the extra pregnancies may be due to the natural fertilisation of extra eggs released after the woman is given fertility drugs, but not harvested by the doctors to be fertilised in vitro.

Provided there is no specific reason which entirely rules out pregnancy, such as blocked fallopian tubes, there is always a small chance of naturally created embryos implanting alongside those produced with IVF.

He said that while it was highly unlikely that sex following implantation could dislodge the embryo, there was still a theoretical chance.

He said: "If intercourse for any reason brought about contractions of the uterus, it could expel the embryo.

"So we advise couples not to be sexually active immediately after IVF."