Sex is a form of biological communication that women use to make decisions about whether to spend energy on pregnancy.
The act of penetrative sex has evolved over millions of years as a mechanism for the delivery of sperm to the eggs and the onset of pregnancy. But sex is more than just a clash of two sets of genes. In the series "Why do you need sex?" we analyze the biological, physical and social aspects of sex and gender. In today's article, we'll take a look at how a woman's immune system responds to sexual intercourse and ensures a healthy pregnancy.
Most people believe that sperm is needed to fertilize a woman's egg and ensure a healthy pregnancy. Therefore, it is generally accepted that all other sperm (and all other sex) are superfluous in terms of the requirements, at least with regard to conceiving a child.
However, biologists today consider sexual intercourse not only as a sperm delivery process, but also as a kind of biological communication. Regardless of whether fertilization takes place, sperm and other components of the ejaculated fluid cause subtle changes in the immune system of women.
This can have a further impact on potential pregnancy. More broadly, sexual activity has implications for pregnancy planning, in vitro fertilization and other forms of assisted reproductive technologies, in the application of which sexual relations and their history are usually left out.
Sperm floats in a soup of molecular information
Evidence from animal studies and clinical studies have led scientists to conclude that semen (the fluid in which semen exists during subsequent ejaculation) plays an important role in fertility.
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Semen contains small molecules that function as biological signals. When injected into the vagina and cervix, they “persuade” the woman’s immune system to accept a “state” that is tolerant to sperm proteins known as “transplant antigens”.
Tolerance is important if the fertilization process is taking place. The immune cells recognize the same transplant antigens in the developing baby and thus support the process by which the embryo attaches to the uterine wall and a healthy placenta and fetus are formed.
Therefore, over time, constant contact with the same male partner stimulates and strengthens a tolerant immune response to his transplant antigens. A woman's immune system reacts to her partner's seminal fluid to gradually increase her chances of forming a healthy pregnancy over at least a few months of regular sex.
Some forms of infertility and pregnancy disorders are caused by immune rejection, when the process of tolerance has not been properly started.
Pregnancy is better after a few months of sex
A condition known as preeclampsia provides us with useful insights into how interactions with seminal fluid affect a successful pregnancy. Preeclampsia is an inflammatory disorder during pregnancy that threatens fetal development and often causes premature birth. It can be life-threatening for the mother if left untreated.
Preeclampsia occurs more often when the number of sexual contacts with the child's father before conception was limited; this condition is associated with insufficient development of immune tolerance in the mother.
The length of time a couple has had sexual intercourse appears to be more important than the frequency of contact. A study on first pregnancies in 2,057 Australian women found that about 5% suffered from preeclampsia. Women with this disorder, compared with women who had a pregnancy without disturbances, were twice as likely to have a short sexual relationship with a partner (less than six months).
Women whose sexual activity with a partner at conception lasted less than three months had a 13% chance of developing preeclampsia, that is, twice as high as the average. Among the few women who conceived at the first sexual contact with the child's father, the likelihood of preeclampsia was 22%, that is, it was three times higher than the average. Low birth weight was also more common in this group.
None of the relationships are analyzed in relation to sexual activity during pregnancy and the risk of preeclampsia, so the duration of the interaction before conception is of the greatest importance.
Formation of immunity tolerance, which promotes a healthy pregnancy, appears to be characteristic of a partner involved in conception. Women who change partners return to their original state, they have to re-develop immune tolerance with a new partner.
Women who use barrier contraceptives, such as condoms or cervical caps (which reduce the interaction of the vagina and cervix with seminal fluid and semen), and subsequently conceive soon after giving up contraception, have an increased risk of developing preeclampsia.
In contrast, those women who use intrauterine contraceptives before conception have a slightly lower risk of developing this condition.
IVF sex can increase the chances of conception
The importance of sex in creating the right conditions for a healthy pregnancy has also been noted in clinical trials with IVF and other assisted reproductive technologies. Fertility is improved when couples have sexual relations during the embryo transfer to the uterus.
Combined data from more than 2,000 patients, collected from seven studies, showed that the chances of pregnancy are increased by 24% after vaginal contact with seminal fluid during oocyte retrieval and embryo replanting. A study involving Australian and Spanish couples found that having sex in the days immediately preceding embryo transfer increased pregnancy rates by 50%.
These studies have focused on the early stages of pregnancy, which requires further research in order to argue that sexual relations affect the entire pregnancy after the use of assisted reproductive technologies.
The lack of interaction with semen may be one of the factors explaining why preeclampsia is observed more often after the use of a donor egg or donor sperm, when no contact of the woman with donor transplant antigens has occurred. The increased risk of using donor semen can be reduced if the same donor is used in previous fertilization cycles.
In couples using a modified version of IVF during conception known as Intracytoplasmic Sperm Injection, the incidence of preeclampsia is also more common in women who have minimal interactions with their partners' transplant antigens due to very low sperm count.
In some couples, an imbalance in the composition of the seminal fluid or features of the immune system can reduce or slow down the formation of a state of immune tolerance in women. Other couples may have immunological incompatibilities that prevent tolerance despite the time spent together.
Some couples may need a longer period of sexual intercourse to become pregnant.
The immune system acts as a gatekeeper during pregnancy
It is interesting to consider why the immune system is so closely related to reproduction.
One theory is that females have evolved to sense and respond to signals in semen to determine the quality or "suitability" of a male partner's genetics. Scientists are now trying to identify the key signals from the male and female sides that promote tolerance.
In addition, since male smoking, being overweight and other factors can influence how a woman reacts to sex in a biological sense. This helps explain why the health of the father is as important as that of the mother in preparation for pregnancy.