Contrary to previous findings - mothers give birth to twins less often

Just under 3 percent of all newborns are twins born.

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 must be because on average these moms give birth more often.

They are even called “Super Moms”, as they should be stronger and healthier.

But now I found an international research group This theory is contradictory.

– It’s very interesting. Twin moms aren’t super fertile super moms, in fact they’re less fertility from other mothers Gine Roll Skjærvø to NRK.

She works in the Department of Biology at NTNU. Together with 13 other researchers, Skjærvø studied about 100,000 European births.

Back to 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 records in Soknedal and from Smola in central Norway.

The aggregate database comes from pre-industrial periods in Norway, Sweden, Finland, Germany and Switzerland. That is, the data is from the years 1700-1899.

At that time, people had normal fertility and a large family size. There were no contraceptives or any other means to control the number of children. There is no assisted reproduction, Skjærvø says.

The risk of death is great

Twin births occur among people all over the world, although there are high health risks associated with them. For both mother and child.

However, this is something that evolution has allowed. Why do researchers ask?

The common explanation was that the risk of death was partially offset by higher fertility. Once all goes well, you have twice as many children who can pass on your genes.

It is also believed that the most fertile women often release two eggs during ovulation. This, in turn, means that the birth of twins is a sign of fertility.

Several researchers have supported this theory. anyway fram Currently.

Researchers have gone “person which is a slang word”

– DrIt was a long process of convincing colleagues that twin mothers could no longer have children, says Gine Roll Skjærvø.

Already in 2010, she says, two researchers in the study began thinking about the idea. They thought that many of them fell into a systematic trap, called Simpson paradoxwhen they researched the subject.

Because by gathering all the twin mothers into one group, one had previously failed to take into account the important lottery effect. DrThat twin mothers are statistically only lucky to have given birth multiple times.

– This may of course seem an issue now at a later time. But no published studies in medicine or evolution have taken into account variation in the number of births when one has examined the relationship between the tendency to have twins and fertility, Skjærvø says.

Mixed cause and effect

In other words, previous research has not been able to tell whether giving birth to twins more often because they are more fertile, or whether giving birth more often increases the chance of a twin birth.

– And so the cause and effect got mixed up, explains the study’s leader, Alexandre Curteau.

He works at the Leibniz Institute in Germany. Curteau now believes that he and his colleagues can prove that women who give birth to twins Not be unusually fertile.

If the mother gives birth often, then most likely one of these births will be twins. Also, you are more likely to win the lottery if you buy more tickets.

This is what co-author Ian Ricard of Durham University in the UK told Gemini.no.

With this effect in mind, one finds that mothers of twins actually give birth less than others. Not often, the researchers point out.

So this should directly conflict with the previous findings.

– wrong interpretation

Per Magnus is a medical geneticist and Director of the Norwegian Institute of Public Health (NIPH).

The findings are believed to be significant, and the new article analyzes the data in a comprehensive manner.

– Previously, the relationship between the total number of births and twin births was interpreted as an expression of causation at the individual level. This is a misinterpretation, he tells NRK.

He believes the results will be important to people who work with fertility.

– It prevents researchers from following the wrong clues, such as studying twin mothers specifically to understand the causes of high fertility.

However, Magnus notes that the new findings will not have direct consequences for clinical work.

It can help with infertility

Gine Roll Skjærvø also believes the new study should be of interest to the health service.

– This will help him get better Knowledge of the relationship between genes and physiological mechanisms related to fertility. In this way, one can again find ways to treat infertility, for example.

The researchers say there are still many unanswered questions, but they believe there are two reasons why evolution has not eliminated twin births.

One is that such births can be the result of two eggs being released during ovulation. This offsets the chance of miscarriage at an advanced age, and benefits younger women.

Secondly, if the risk of death from giving birth to twins is not very high, then having twins with larger families. Although it turns out that women who have twins give birth less often.