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Study Finds the Time Frame Morning Sickness Is Most Likely to Start

“The precise course of pregnancy sickness is unknown, but this research shows that it occurs at a specific developmental stage, in a specific time slot. For researchers it narrows our focus in terms of where we look for the cause.”
ByNehal Aggarwal
Associate Editor
Published
July 14, 2021
Woman laying on couch in pain, experiencing morning sickness.
Image: Alla Bielikova / Getty Images

Morning sickness is certainly one of the less fun aspects of pregnancy, especially since, unlike the name implies, it can strike at any time. But, while it’s a really common aspect of pregnancy, experts aren’t entirely sure what causes it. They believe it likely is related to a rush of hormones, including Human Chorionic Gonadotropin (hCG), which lets your body know you’re pregnant. Now, one study has pinpointed the time frame in which morning sickness is likely to start and may be closer to determining a cause.

The study, conducted by researchers at the University of Warwick and published in the journal BMC Pregnancy and Childbirth, looked at data, collected at the Clearblue Innovation Centre, from the daily symptom diaries of 256 pregnant women. They compared when the nausea started for each and their vomiting symptoms to the date of their last menstrual period, which is more commonly used to measure the start of pregnancy, and the date of ovulation, which is thought to be a more accurate starting point and determined by a urine test.
When using the date of ovulation, they found that most of the pregnant women began experiencing symptoms of morning sickness after 8 to 10 days, as compared to 20 to 30 days if they used the date of the last menstrual period. They believe that using the ovulation date not only pinpoints the timeframe that symptoms start, but also shows that symptoms may start sooner than previous research has shown.

"The precise course of pregnancy sickness is unknown, but this research shows that it occurs at a specific developmental stage, in a specific time slot. For researchers it narrows our focus in terms of where we look for the cause. If we know that symptoms occur in a very narrow window 8-10 days after ovulation, researchers can concentrate their efforts on that particular stage of development to find the cause of the condition, both anatomically and biochemically,” lead author and professor Roger Gadsby of Warwick Medical School said in a release. “In the past, women suffering with nausea and vomiting in pregnancy have had their symptoms trivialised and overlooked because it was thought there was a psychological basis for the symptoms. This research further reinforces that nothing could be further from the truth, that this is a biological problem related to the development of the early fetus.”

For most women, morning sickness starts to end by 12 to 14 weeks of pregnancy, but some experience it more severely, as is the case in hyperemesis gravidarum, in which the symptoms can continue throughout the pregnancy.

The researchers also found that 94 percent of women experienced symptoms of morning sickness, which was higher than the 80 percent previous research calculated. They believe this likely because they used data from all the participants from up to 60 days after their last menstrual period, while previous studies may have asked pregnant women to recall symptoms after they were already pregnant.

“What we’ve shown is that more people get symptoms of pregnancy sickness than has ever been shown before, and one of the reasons for that is that this research has picked up mild early symptoms that tend to fade by 7-8 weeks,” Gadsby said. “In other studies those symptoms would have faded by the time the research started.”

Previous research from the same team has also concluded that the term “morning sickness” should be replaced with “pregnancy sickness,” as it can occur at any time of the day and avoids trivializing the condition.

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