A quarter of miscarriages may be preventable, research suggests.
Being underweight or obese before conception, working nights, drinking alcohol in pregnancy and lifting heavy loads could all increase the risk, experts said.
They said if women cut these risks to very low levels, 25 per cent of miscarriages could be prevented.
The large study of more than 91,000 pregnancies also confirmed that one of the biggest factors influencing miscarriage is the mother’s age, with women over 30 having a higher chance of miscarrying.
However, other researchers warned the study did not show miscarriage was “caused” by any of the factors and pointed to other limitations.
The team behind the study, from the University of Copenhagen in Denmark, used data from 91,427 Danish pregnancies between 1996 and 2002.
They regarded miscarriage as before 22 weeks of pregnancy and found that 3,177 of the total sample of pregnancies resulted in miscarriage.
Women were asked during a telephone interview to discuss their habits leading up to conception and during pregnancy.
The women were typically interviewed around 16 weeks into their pregnancy. Those who had already suffered a miscarriage were asked about their habits leading up to the miscarriage.
The study found that age, drinking alcohol, lifting more than 20kg a day, night shifts and being obese or overweight all contributed to miscarriage.
A mother’s age and alcohol consumption were the most important risk factors.
Writing in BJOG: An International Journal of Obstetrics and Gynaecology, the experts said: “In a prevention scenario where the women conceived at an age of 25 to 29 years, consumed no alcohol during pregnancy, were normal weight before pregnancy, did not lift more than 20kg daily during pregnancy, and worked only during the day, 25.2 per cent of the miscarriages were preventable.
“This scenario resulted in the highest preventable proportion of miscarriage.”
The researchers estimated that 11.4 per cent of miscarriages could be prevented if women conceived aged 25 to 29, and nine per cent of miscarriages could be prevented if no alcohol was consumed during pregnancy.
Sandra Feodor Nilsson, a PhD student from the Department of Public Health at the University of Copenhagen and co-author of the study, said: “Miscarriage is the most common adverse pregnancy outcome affecting at least one in seven pregnancies and is considered irreversible. Therefore, prevention may be the only way of reducing the number of miscarriages that occur.”
According to the NHS Choices website for patients, one in seven pregnancies in the UK end in miscarriage and most are due to chromosomal abnormalities in the baby.
If a miscarriage happens during the second trimester of pregnancy (between weeks 14 and 26), it says an underlying health condition in the mother may be to blame.
However, women can reduce their risk by avoiding smoking, drinking alcohol and using drugs while pregnant, and keeping to a healthy weight, it said.
Professor Tom Bourne, a consultant gynaecologist at Queen Charlotte’s and Chelsea Hospital in London, said: “This is a big study, but it does not really say anything new.
“There are also issues of recall bias, and the fact that they show an association rather than causation. Saying that by changing X or Y a percentage of miscarriages could be prevented is quite a statement in the absence of an interventional trial.
“However, it adds to the view that alcohol in pregnancy is not a good idea, and we know miscarriage increases with age in any event. The association with lifting and night work has been reported before.”
Patrick Wolfe, professor of statistics at University College London (UCL), said: “This study does not establish a causal relationship between its reported risk factors and miscarriage.
“As the commentary that follows it notes, let’s not run before we can walk. The study has several statistical limitations, and so I caution that its conclusions may be subject to over-interpretation. It’s best to think of this study as identifying potential candidate risk factors that may be associated with miscarriages that occur later in pregnancy, rather than the last word on the subject overall.”