Wednesday July 30 2014
Around half of pregnant women develop morning sickness
“Morning sickness isn't all bad news: Women battling the condition may have 'healthier, more intelligent babies’,” the Mail Online reports.
The news is based on the results of a systematic review that looked at the effects of “morning sickness”. Health professionals prefer the term “nausea and vomiting in pregnancy (NVP)” because, as many pregnant women can attest, symptoms can occur at any time.
The researchers were interested in whether NVP was associated with better pregnancy outcomes.
The researchers identified 10 observational studies. All 10 reported a protective effect of NPV, such as a reduced risk of miscarriage, birth defects and premature birth. There was also evidence of an association with better foetal growth and improved mental development.
The results of this study may reassure women with NVP that it could be a good sign.
However, it is difficult to answer exactly why and how NVP may be associated with improved pregnancy outcomes. One hypothesis is that heightened hormonal levels may help the baby; however, they also make the mother feel terrible.
Before pregnant women with NVP get too excited that they are carrying a genius, it’s important to note that the link with intellectual developmental outcomes come from a single, small study – this is hardly conclusive evidence.
Where did the story come from?
The study was carried out by researchers from The Hospital for Sick Children, and the University of Toronto, Canada. Funding sources for this study were not reported, but the researchers state that there are no conflicts of interest.
The study was published in the peer-reviewed journal Reproductive Toxicology.
The results of the study were generally well reported in the UK media. However, it should be noted that as the systematic review did not pool the results of the included studies, some of the figures reported in the media are the results of individual studies.
The Daily Telegraph's headline of “intelligent babies” doesn’t seem so conclusive when you realise this is only based on a single, small study including 45 treated and 47 untreated pregnant women with nausea and vomiting, and just 29 pregnant women without.
What kind of research was this?
This was a systematic review of cohort and case-control studies that compared rates of any adverse pregnancy outcome (such as miscarriages, preterm delivery, intrauterine growth restriction, major congenital malformations and long-term developmental outcomes) between women who experienced NVP and women who did not.
In this systematic review, the results of the studies were not pooled; instead, the results of each study were described separately.
All the studies in this systematic review were observational. This means that there could be a wide variety of confounding biological, health and lifestyle differences between women and pregnancies associated with NVP and those without, which could explain the results.
What did the research involve?
The researchers searched databases of published literature to identify cohort and case-control studies that had compared rates of any adverse pregnancy outcomes between women who experienced NVP and women who did not.
Outcomes searched for included miscarriages, preterm delivery, intrauterine growth restriction (unusually low birthweight) major congenital malformations and long-term developmental outcomes. Some of these studies identified women with NVP by the use of drugs that control these symptoms, such as antiemetics (a type of drug that is used to treat vomiting and nausea).
Data on the relationship between NVP and pregnancy outcomes was then extracted, along with other study characteristics.
What were the basic results?
10 studies were included in the review. Some of the studies reported on more than one pregnancy outcome.
No study was included that did not report a protective effect of NVP.
Two cohort studies examined the relationship between NVP and miscarriage. Both studies found that NVP reduced the risk of miscarriage. One US cohort study found that lack of NVP symptoms was associated with increased risk of miscarriage. An Ecuadorian cohort study found that women with nausea only or nausea with vomiting had a significantly decreased risk of miscarriage.
Six studies examined the relationship between NVP and congenital malformations (malformations present from birth).
All of the studies found that NVP reduced the risk of congenital malformations.
One Hungarian case-control study found that mothers of cases with congenital abnormalities were less likely to have had severe NVP than the mothers of population controls without congenital abnormalities.
Three Swedish case-control studies were included: one found that babies born to women who used any antiemetic had a reduced risk of congenital malformations.
One found that meclozine use (an antihistamine that is considered to be an antiemetic) was associated with a reduced rate of congenital malformations.
The final Swedish study found a reduction in the occurrence of congenital cardiovascular defects associated with the use of antihistamines for NVP.
One study was a meta-analysis of 24 studies. It found that antihistamines taken during the first trimester (12 weeks) reduced the risk of major malformations.
One US case-control study found that NVP was associated with a reduced risk of cleft lip with or without cleft palate, and hypospadias (an abnormality of the urethra and penis).
Four studies looked at the relationship between NVP and growth restriction. All three Swedish case-control studies reported on these outcomes. One found that antiemetic drug use was associated with a reduced risk of low birthweight, prematurity and being small for gestational age; another found that meclozine use was associated with reduced risk of preterm birth, low birthweight and short body length; another found that pregnancies exposed to antihistamines for NVP had a reduction in preterm birth, low birthweight and being small for gestational age.
The Hungarian cohort study found that mothers with NVP symptoms had a lower proportion of preterm births.
One study looked at the relationship between NVP and long-term child development. If found that children exposed to NVP performed better on some psychological tests.
How did the researchers interpret the results?
The researchers conclude that “the present analysis reveals a consistent, favourable effect of [NVP] on rates of miscarriages, congenital malformations, foetal growth, prematurity and better development outcomes on standard psychological tests”.
The researchers note that this finding may have an impact on other studies – notably, observational studies of drug safety in pregnant women. This is because differing proportions of women with NVP in the exposed and non-exposed groups could affect the results.
This systematic review has found that nausea and vomiting of pregnancy is associated with favourable pregnancy outcomes.
All 10 observational studies included in the review reported a protective effect of NVP.
NVP was associated with a reduced risk of miscarriages, congenital malformations and prematurity, and better developmental outcomes.
Beneficial associations were seen, even if women took medication to control their symptoms.
The results of this study may help to reassure women with nausea and vomiting in pregnancy that this may be a good sign – at least for some measures of a healthy, ongoing pregnancy.
Overall, however, due to the small number of observational studies included, the small number of outcomes examined and the uncertain nature of the relationship and possibility of confounding, the review does not prove that NVP will always indicate a healthy pregnancy.
Importantly, it does not prove the opposite – that a pregnancy without nausea and vomiting means a poorer outcome.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.