A research team analyzed records of nearly a million women in Sweden’s national medical registries from 2001 through 2017, comparing 86,551 women who had perinatal depression with 865,510 women who did not. The groups were matched by age and year they gave birth.

In two studies, the team found that depression that begins in pregnancy or soon after can have troubling implications for as long as 18 years.

One study, published on Tuesday in JAMA Network Open, found that women with perinatal depression had three times the risk of suicidal behavior, defined as attempted or completed suicide. Risks were greatest in the year following their diagnosis, but, while they lessened over time, years later the risks were still twice as high compared with women without the disorder.

The other study, published on Wednesday in BMJ, found that women with perinatal depression were more than six times at risk of dying by suicide as those without that diagnosis. The number of suicides was small, but it accounted for a large share of the deaths of women diagnosed with perinatal depression: 149 of the 522 deaths in that group, or 28.5 percent. For women without perinatal depression, there were 117 suicides out of 1,568 deaths or 7.5 percent.

Suicide was a major reason women with perinatal depression were twice as likely to die from any cause over the 18-year period of the study compared with women without the disorder.

The researchers also compared more than 20,000 women with perinatal depression to their biological sisters who gave birth during the same time frame and did not have the disorder. The risk of suicidal behavior for the sisters with perinatal depression was nearly three times that of their sisters without the diagnosis — almost as high as the difference between women with the illness and those without it to whom they were not related. That suggests depression plays a greater role in these outcomes than genetics or childhood environment, the researchers wrote.

The average age at which women experienced perinatal depression was 31. They were more likely than those without the illness to live alone, to have lower income and less formal education, to have smoked recently and to have not given birth before, among other characteristics, the researchers reported.

They were also more likely to have had previous psychiatric disorders or suicidal behavior. But the studies found that regardless of whether women had other mental health problems, perinatal depression increased the risk of suicidal behavior and death. That suggests that pregnancy-related depression can be different and more severe than other mental health disorders.

“This highlights the pressing need for vigilant clinical monitoring and prompt intervention for this vulnerable population to prevent such devastating outcomes, regardless of pre-pregnancy history of psychiatric disorders,” the JAMA Open Network study concluded.

There may be differences in the experience of women who develop depression during pregnancy — just over half of those in the study — and those with postpartum depression that emerges within a year after giving birth. The researchers found that women with postpartum depression were at greater risk for suicidal behavior and death, but it was unclear why.

The data did not find that pregnancy complications or a newborn’s birth weight or gestational age affected a mother’s suicidal behavior.

Perinatal depression, estimated to affect 10 to 20 percent of women during or soon after pregnancy, remains understudied, under-treated and probably underdiagnosed. Some reports have suggested that the disorder became even more common during the coronavirus pandemic because of social isolation and diminished access to care.

While the new studies shed some light on the effects of perinatal depression, many questions remain. For example, the researchers said they could not capture factors like domestic violence or alcohol consumption. And the studies may not be representative of experiences in other countries, since, for example, most women in the Swedish registries were white and Sweden is relatively affluent with universal health care.

The role of treatment and therapy is not yet fully understood. The studies classified women as having perinatal depression if doctors gave them that diagnosis or they had filled prescriptions for antidepressants during pregnancy or one year later, indicating that they received some treatment. For those without treatment or therapy, the consequences could be magnified.

If you are having thoughts of suicide, call or text 988 to reach the 988 Suicide and Crisis Lifeline or go to SpeakingOfSuicide.com/resources for a list of additional resources. Go here for resources outside the United States.