I didn't know that it's now believed that exposure to semen prepares a woman's body immunologically for pregnancy:
Seminal fluid contains small molecules that act as biological signals. Once deposited in the vagina and the cervix of a woman, these persuade the woman’s immune system to adopt a profile that tolerates (that is, recognises and accepts) sperm proteins known as “transplantation antigens”.And here's some strong sounding evidence to back this up:
The tolerant profile matters if fertilisation takes place. Immune cells recognise the same transplantation antigens on the developing baby, and so support the process through which the embryo implants into the wall of the uterus and forms a healthy placenta and fetus.
So over time, repeated contact with the same male partner acts to stimulate and strengthen a tolerant immune response to his transplantation antigens. The immune system of a woman responds to her partner’s seminal fluid to progressively build the chances of creating a healthy pregnancy over at least several months of regular sex.
Preeclampsia is more common when there has been limited sexual contact with the father before pregnancy is conceived, and is associated with insufficient establishment of immune tolerance in the mother.Although its frequency seems not so important for preeclampsia, the article notes that sex around the time of using IVF does help:
The length of time a couple have had a sexual relationship seems more important than the frequency of intercourse. In a study of first pregnancies in 2507 Australian women, around 5% developed preeclampsia. Affected women were more than twice as likely to have had a short sexual relationship (less than six months) compared to the women who had healthy pregnancies.
Women with less than three months sexual activity with the conceiving partner had a 13% chance of preeclampsia, more than double the average occurrence. Among the few women who conceived on the first sexual contact with the father, the chance of preeclampsia was 22%, three times higher than the average. Low birth weight babies were also more common in this group.
Combined data from more than 2000 patients across seven studies showed the occurrence of a detectable pregnancy increased by 24% after vaginal contact with seminal fluid near the time of egg collection or embryo transfer. A study of Australian and Spanish couples showed intercourse in the days just before or just after embryo transfer boosted pregnancy rates by 50%.I guess this suggests that couples who want children in the future may be better off in the long run to not rely on barrier methods only as a contraception. Good news for men, at least...