Women who have mental health problems have an almost 50% higher risk of preterm births, the biggest study of its kind revealed.

According to the study, which was released on Tuesday in the Lancet Psychiatry and looked at data from more than 2 million pregnancies in England, one in 10 women who had used mental health services had a preterm birth, compared to one in 15 of those who had not.

The study also discovered a direct relationship between bad delivery outcomes and the severity of prior mental health issues. Compared to women who had no prior interaction with mental health services, women who had previously been admitted to a psychiatric institution were nearly twice as likely to give birth prematurely.

In addition, mothers with a history of mental health issues had a higher risk of giving birth to a child who was small for gestational age (75 births per 1,000 versus 56 births per 1,000).

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The study, in which researchers from King's College London, the University of Exeter, the London School of Hygiene & Tropical Medicine, and the University of Liverpool took part, suggests that detailed questions regarding mental health should be directed to pregnant women when they are first seen by doctors and midwives.

Such screening would aid in identifying "clear red flags for a possible adverse outcome," according to Louise Howard, a King's College London professor emerita and one of the report's authors.

She stated that once these mental health risk factors are identified, they can be treated, potentially lowering the number of maternal fatalities, stillbirths, early births, and underweight kids.

"Mental illness is a treatable problem … maternity professionals will be focused on thinking about other modifiable risk factors such as smoking and obesity, but they may not have thought about some of the additional risks that women with mental illness may have," she said.

She said the study demonstrated the requirement for well-funded mental health services.

"These services are under huge pressure," she said. "But this study highlights the need for these different interventions to be available, because they really do make a difference."

The Nice guidelines on prenatal and postnatal mental health, which were developed in 2014, according to Howard, should be modified in light of the study.

She said that "what we didn’t emphasize in 2014, because we didn’t have the data then, is how important it is to take a really detailed history of when mental health problems occurred and how severe they were," adding that the significance of the study is associating "adverse birth outcomes with the severity of illness. It also emphasizes the need for really good training of midwives and obstetricians around mental health issues."

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In contrast to 0.45% of women who had no pre-pregnancy mental health care, the study indicated that 0.65% of pregnant moms who had been admitted to a psychiatric institution suffered a stillbirth.

Negative birth outcomes and mental health have also been linked in earlier, smaller research. However, Howard claimed that the size of this investigation made a difference. 

The study did not investigate the mechanisms underlying the association between poor mental health and unfavorable pregnancy outcomes.

"Mental illness itself is likely to be associated with some of these adverse outcomes, because it impacts the stress system and is therefore likely to cause problems. But it is also associated with other factors. If you’ve got mental illness, you’re more likely to smoke because you’re feeling stressed. Similarly with substance misuse. And there are other stressors such domestic violence, or a history of childhood abuse, or poverty," Howard said.

She added that issues during pregnancy may also be related to mental health treatments.

"Antipsychotics are associated with obesity, and we know that obesity is a major risk factor for these outcomes. Some antidepressants may be associated with some of these outcomes as well. That’s why we have to think really carefully about who would benefit from medication or whether psychological therapy could be used instead."