New mothers who start hormonal contraception after giving birth may face a slightly increased risk of depression in the postpartum period, according to new research.

The research – coming from a contentious population-based study in Denmark – is only observational, which means it cannot prove that hormonal contraception caused the outcomes.

But the potential for an "instantaneous" increased risk of depression is one that doctors and new mothers should be aware of, according to the authors, led by neurobiologist Søren Vinther Larsen from Copenhagen University Hospital–Rigshospitalet.

In a cohort of more than 600,000 first-time mothers, researchers found those who used hormonal birth control methods within a year of giving birth had nearly 1.5 times the risk of being diagnosed with depression in hospital or being prescribed antidepressants compared to non-users.

Combined oral contraceptives, which contain both progesterone and estrogen, were particularly tied to depression in new mothers.

Compared to those not using hormonal contraceptives, new mothers who took the combined birth control pill were 1.7 times more likely to experience depression within a year of delivery.

The sooner 'the Pill' was started postpartum, the greater the risk.

While this might sound concerning, it's important to consider the absolute risk for an individual, not just the relative risk between groups.

The absolute risk was 1.54 percent for pill users and 1.36 percent for non-users. That's a difference of just 0.18 percentage points. And the way that depression was measured in the study may not accurately reflect actual depressive symptoms.

"Antidepressants may be prescribed for a range of indications, not only depression," argues reproductive health researcher John Reynolds-Wright, who was not involved in the current study.

"Changing advice about postpartum contraception is not supported."

It's a small risk, if it exists at all, especially when weighed against the benefits of hormonal contraception, which are backed by much more evidence. That said, it is a risk that some think has been "inadequately addressed", historically.

As we start to get new results, however, it's important to interpret them cautiously.

Depression is one of the most common side effects reported by users of hormonal contraceptives.

Clinical trials that have investigated the link between depression and hormonal contraceptives have generally failed to turn up a connection.

However, some recent observational studies have found that combined oral contraceptives are associated with an increased risk of depression shortly after initiation. Over time, this risk seems to plummet, which suggests the body gets used to the hormones.

Individuals with other risk factors for depression may be more impacted though, and that would be important to know both for physicians and patients.

It's possible, for instance, that new mothers with a history of a hormonal-contraceptive-associated mood deterioration, premenstrual dysphoric disorder, or postpartum depression may face a greater risk of depression associated with hormonal birth control in the postpartum period.

But despite such sound logic, the new study found those without a history of prior mental health disorders were at an even greater risk of depression following the use of hormonal contraception.

The study from Denmark is a sweeping analysis, but it fails to consider some individual factors that could be influencing the results.

Women who gave birth previously were not considered, and neither were women who had a history of depression or antidepressant usage in the two years before becoming a mother.

"Considering both of these aspects may have altered the findings of the study," warns Reynolds-Wright, who works at the University of Edinburgh in the UK.

Interestingly, the study found that age did not impact the risk of depression following hormonal contraception. Previous observational studies suggest adolescents are at an increased risk, but it's unknown why: it could be because their brains are still developing, or because they are first-time users, still getting used to the hormones.

Perhaps the postpartum period is another "window of higher susceptibility", speculate Larsen and their colleagues. At this stage, the brain has gone through profound brain changes and the body is readjusting to new hormone levels.

"This raises the issue of whether the incidence of depression postpartum is increased by routine hormonal contraceptive initiation after childbirth," suggests the team of researchers.

That issue has been raised, but by no means has it been solved.

The study was published in JAMA Network Open.