The act of penetrative sex has evolved over millions of years as a mechanism to deliver sperm to eggs and initiate pregnancy. But there's more to sex than just the meeting of two sets of genes. The Conversation's "What's the point of sex?" series examines biological, physical and social aspects of sex and gender.
Today's piece looks at how a woman's immune system responds to sexual intercourse and facilitates healthy pregnancy.
Most people think just one sperm is needed to fertilise a woman's egg and make a healthy pregnancy. This underpins a common view that all the other sperm - and all the other sex - are surplus to requirements, at least when it comes to conceiving a pregnancy.
However, biologists now believe sexual intercourse is not just a sperm delivery process, but also a kind of biological communication. Regardless of whether fertilisation occurs, sperm and other components of the ejaculated fluid trigger subtle changes in the immune system of women.
This has consequences for pregnancy should it happen later. More broadly, the importance of regular sexual activity also has implications for fertility planning, and for IVF and other forms of assisted reproduction, which generally do not take sexual practice or history into account.
Sperm swim in a soup of molecular messages
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".
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.
Some forms of infertility and disorders of pregnancy are caused by immune rejection, when the process of tolerance is not adequately established.
Healthier pregnancy after months of sex
A condition known as preeclampsia provides useful insights into how exposure to seminal fluid influences the success of pregnancy. Preeclampsia is an inflammatory disorder of pregnancy that compromises growth of the fetus, and often causes prematurity in babies. It can be life-threatening for mothers if left untreated.
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.
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.
No relationship is observed between frequency of sexual activity during pregnancy and risk for preeclampsia, so it's the duration of exposure before conception that counts most.
Setting up a profile of immune tolerance that supports healthy pregnancy seems to be specific to the conceiving partner. Women who change partners return to a baseline state, and must rebuild immune tolerance with the new partner.
Women who use barrier methods such as condoms or cervical caps (which lower the exposure of the vagina and cervix to seminal fluid and sperm), and then conceive shortly after stopping contraception, have an elevated risk of preeclampsia.
Sex during IVF can increase conception chances
The importance of sex in creating the right environment for healthy pregnancy is also observed in clinical studies in IVF and other methods of assisted reproduction. Fertility is improved when couples have intercourse in the period when an embryo is transferred to the uterus.
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%.
These studies focused on the early stages of pregnancy, with further research required to assess whether sexual intercourse influences rates of full term pregnancy after assisted reproduction.
Absence of exposure to seminal fluid may be one factor explaining why preeclampsia incidence is higher after use of donated eggs or donor sperm, where prior female contact with the donor transplantation antigens has not occurred. The elevated risk after using donor semen can be reduced if multiple prior insemination cycles take place with the same donor.
In couples who conceive using a modified version of IVF known as ICSI (intracytoplasmic sperm injection), preeclampsia incidence is also higher in women who experience minimal exposure to their partner's transplantation antigens due to very low sperm counts.
In some couples, an imbalance in seminal fluid composition or immune system factors may inhibit or slow down establishment of the tolerant immune profile in women. In other couples, there may be immunological incompatibility that impairs tolerance, regardless of time spent together.
Maybe some couples may just need a little longer having sex for pregnancy to occur.
Immune system acts as a gatekeeper in pregnancy
It is interesting to consider why the immune system is so closely involved in reproduction.
One theory is that females have evolved the ability to sense and respond to the signals in seminal fluid, in order to discern the quality or "fitness" of the male partner's genetics. Scientists are now seeking to define the key signals on the male and female sides that promote tolerance.
Also, since male smoking, being overweight and other factors may shape how a woman responds to intercourse in a biological sense, it helps explains why dad's health is just as important as that of the mother in preparing for pregnancy.