Just under 3 percent of all births are twin births. In other words – they are not very common. Previous studies have concluded that mothers who give birth to twins are more fertile than others. This should be because these mothers on average give birth more often. They have even been called “supermothers”, as they should be more hardy and have better health. But now an international research group has found that this theory is jarring. – It’s quite interesting. Twin mothers are not supermoms who are very fertile, they actually have lower fertility than other mothers, says Gine Roll Skjærvø to news. She is employed at the Department of Biology at NTNU. Together with 13 other researchers, Skjærvø has studied approximately 100,000 European births. Back to the pre-industrial society The new study is a major collaboration across several countries, and has been published in Nature Communications. In Norway, Skjervø and a colleague at NTNU have collected data from church registers in Soknedal and from Smøla in central Norway. The total data base comes from pre-industrial times in Norway, Sweden, Finland, Germany and Switzerland. That is, the data is from the years 1700–1899. – At that time, people had natural fertility and large family sizes. There was no contraception or other method to control the number of children. No assisted reproduction, says Skjærvø. High risk of death Twin births occur among people worldwide, although there are high health risks. Both for mother and child. Yet this is something evolution has allowed. Why, the researchers ask? A common explanation has been that the risk of death has been partially offset by high fertility. Once things go well, you have twice as many children who can pass on your genes. It has also been thought that women who are more fertile more often release two eggs during ovulation. Which in turn means that giving birth to twins is a sign of fertility. Many researchers have supported this theory. At least until now. Researchers have fallen into the “trap” – It has been a long process to convince colleagues that twin mothers are no longer fertile, says Gine Roll Skjærvø. She says that already in 2010, two of the researchers in the study started thinking about the idea. They thought several had fallen into a methodical trap, called Simpson’s Paradox, when they investigated the topic. Because by gathering all twin mothers in one group, one has previously failed to take into account an important lottery effect. The fact that twin mothers are only statistically lucky because they give birth several times. – This may seem a matter of course now in retrospect. But there are no published studies in medicine or evolution that have taken into account the variation in the number of births when one has studied the connection between the tendency to have twins and fertility, says Skjærvø. Gine Roll Skjærvø is a human behavioral biologist at the Department of Biology at NTNU in Trondheim. Photo: Private – Has mixed cause and effect In other words, previous research has not been able to tell whether twin mothers give birth more often because they are more fertile, or whether giving birth more often increases the chance that a twin birth can occur. – So there has been a mixture of cause and effect, explains the leader of the study, Alexandre Courtio. He is employed by the Leibniz Institute in Germany. Courtio now believes that he and his colleagues can prove that women who give birth to twins are not unusually fertile. – If a mother gives birth more often, it is more likely that one of these births is a twin birth. Just as it is more likely that you will win the lottery if you buy more tickets. This is what co-author Ian Rickard from Durham University in the UK tells Gemini.no. Taking this effect into account, one finds that mothers of twins actually give birth less often than others. Not more often, the researchers point out. This must therefore be in direct conflict with previous findings. Several church books were reviewed to find data for the study. Photo: University of Turku – A misinterpretation Per Magnus is a specialist in medical genetics and director of the National Institute of Public Health (NIPH). He believes the findings are important, and that the new article analyzes the data in a thorough way. – In the past, the connection between the total number of births and having had a twin birth has been interpreted as an expression of a causal connection on an individual level. This is a misinterpretation, he says to news. He believes the results will be important for people who work with fertility. – It prevents researchers from following the wrong clues, for example that studying twin mothers specifically to understand the causes of high fertility. Magnus nevertheless points out that the new findings will not have direct consequences for clinical work. Per Magnus believes the new findings will be helpful for people who work with fertility. Photo: FHI Can help against infertility Gine Roll Skjærvø also believes that the new study should be of interest to the health service. – This will help to improve the knowledge about the link between the genes and the physiological mechanisms related to fertility. In this way, one can again find methods for treating, for example, infertility. The researchers say there are still many unanswered questions, but they believe there are two reasons why evolution has not removed twin births. One is that such births can be a consequence of two eggs being released during ovulation. This compensates for the chance of miscarriage at an advanced age, and is beneficial for younger women. Secondly, if the risk of death from twin births is not too high, having twins with larger families. Although it turns out that women with twins give birth less often.
ttn-69
Contrary to previous findings – twin mothers give birth less often – news Trøndelag
