No one would put their hand up to experience the nausea and vomiting that comes with morning sickness, but a new study suggests that there could be a hidden benefit to all that discomfort.
Scientists have found the strongest evidence yet that vomiting and nausea during pregnancy are associated with a lower risk of miscarriage – backing up the popular myth that morning sickness is a sign that the baby is developing well.
"It's a common thought that nausea indicates a healthy pregnancy, but there wasn't a lot of high-quality evidence to support this belief," says epidemiologist Stefanie N. Hinkle from the US National Institutes of Health.
"Our study evaluates symptoms from the earliest weeks of pregnancy, immediately after conception, and confirms that there is a protective association between nausea and vomiting and a lower risk of pregnancy loss."
While previous studies have investigated the relationship between morning sickness and miscarriage rates, Hinkle says this is the first time that nausea and vomiting symptoms have been examined in detail during the earliest weeks of pregnancy – during which up to 80 percent of women can experience morning sickness.
The researchers analysed data taken by the Effects of Aspirin in Gestation and Reproduction (EAGeR) trial, which studied whether low aspirin doses might reduce the rate of miscarriage in 797 women who had experienced one or two prior pregnancy losses.
The participants – all of whom had registered a positive pregnancy test at the time of the trial – kept daily diaries of whether they experienced nausea or vomiting from the second week of their pregnancies up until the 8th week.
From the 8th on, they recorded their symptoms in a monthly questionnaire until the 36th week of pregnancy.
In the trial, 188 of the 797 pregnancies ultimately ended in loss. But the researchers found that women who experienced nausea by the 8th week (57.3 percent of the group) were 50 percent less likely to experience a pregnancy loss than those who didn't have nausea.
And women who experienced both nausea and vomiting by the 8th week (26.6 percent) were 75 percent less likely to experience a miscarriage than those who didn't have both symptoms.
Compared to previous studies that didn't examine in detail women's symptoms during the first eight weeks of pregnancy, the data could prove to be very valuable. The researchers also took into account various factors that could influence miscarriage rates, such as alcohol intake and chromosomal abnormalities in the foetus.
"These findings overcome prior analytic and design limitations and represent the most definitive data available to date indicating the protective association of nausea and vomiting in early pregnancy and the risk for pregnancy loss," the researchers explain in their paper.
That said, it's worth bearing in mind that all of the woman who took part in the study had already experienced one or two miscarriages – something which can't be said for every woman who wants to have a baby, and which might make it difficult to interpret the results with the 'average woman' in mind.
The researchers also acknowledge that their medium-sized study "was conducted within a rather homogeneous sample of women, which may limit generalisability".
It's also worth pointing out that all studies like this rely on participants accurately self-reporting their symptoms too.
But with those limitations in mind, there's clearly a correlation between morning sickness and lower miscarriage rates here, and the researchers aren't sure how to explain it.
"We hypothesise that there is a more direct biological link going on between nausea and vomiting and pregnancy loss," Hinkle told Alice Park at Time, "although our data can't inform exactly what that is."
In their study, the team points out that morning sickness symptoms "may be part of an evolutionary advantage to change one's dietary intake, increase consumption of carbohydrate-rich foods, or avert intake of potentially teratogenic substances".
It's also possible, the researchers say, that morning sickness's ties to miscarriage could somehow be related to the functioning of the hormone human chorionic gonadotropin (hCG) – or that nausea and vomiting are markers for viable placental tissue.
"Thus, less nausea and vomiting may identify failing pregnancies, with lower hormone levels leading to nausea and vomiting," they write.
At this point, more research is needed to figure out the possible biological causes behind this apparent link, but the team is hopeful that their findings may provide some comfort to pregnant mothers who are having a hard time – particularly in the early stages.
"It's a hard time the first time you get pregnant, and then you throw on feeling sick and exhausted," Hinkle told Time. "This should provide some solace and reassurance to women experiencing these symptoms that they have a healthy pregnancy."
By the same measure, the researchers are eager to point out that there's nothing wrong with not experiencing morning sickness – in other words, nobody should feel that they're missing out on some kind of 'protective effect', because researchers are only just starting to look into what might be going on with this association.
"Not all pregnancies are the same, and every individual is different," Hinkle told Adrienne LaFrance at The Atlantic. "So just because they do not have symptoms, does not mean that they will go on to have a loss."
The findings are published in JAMA Internal Medicine.