Persistent fatigue immediately following birth may be the best signal to determine whether a woman will develop postpartum depression, a new study suggests.

Women who said they still felt extremely fatigued two weeks after having a baby were more likely to suffer from postpartum depression a month after giving birth.

“All mothers are tired right after having a baby - it helps them get the rest that they needs to recover and heal from the physical and mental stressors of childbirth,” said Elizabeth Corwin, the study’s lead author and an associate professor of nursing at Ohio State University. “But for most women, fatigue steadily fades within the first two weeks of giving birth.

What is unusual - and detrimental - is a fatigue that persists, the researchers say.

In this study it was fatigue - not stress or a history of depression - that was the best indicator of which women went on to develop postpartum depression.

“For these women, the constant fatigue came first, and depression followed,” Corwin said.

The study appears in a recent issue of the Journal of Obstetric, Gynecological and Neonatal Nursing.

Corwin and her colleagues recruited pregnant women who were near the end of their third trimester. A total of 31 women completed the study. Each woman carried her baby to a full term and delivered vaginally without complications.

A researcher met each woman at her home when she was between 36 and 38 weeks pregnant. The participants were asked to complete questionnaires on fatigue, stress and both symptoms and history of depression. They were also asked to provide a saliva sample which the researchers used to measure levels of cortisol, a hormone related to stress.

Each woman was asked to call one of the researchers as soon as possible after giving birth. At that point, the researcher arranged to visit the woman about seven days after she delivered. The researcher also visited that same woman two weeks and four weeks after delivery.

At each postpartum visit, the women filled out the same questionnaires on depressive symptoms, stress and feelings of fatigue and also provided saliva samples.

By the end of the fourth week, 11 of the 31 women in the study showed symptoms of depression - seven of these women had a family history of depression. Of this group of seven women, four also had a personal history of depression.

And 10 of the 11 women (91 percent) who showed symptoms of postpartum depression during the fourth and final week of the study had also reported higher-than-normal levels of fatigue two weeks earlier. Only one of the 11 women who went on to demonstrate symptoms of depression had not reported excessive fatigue at that visit.

“A personal history of depression is an excellent way to predict which women are at risk for postpartum depression,” Corwin. “Still, using that as the sole screening tool would have left seven of the women undiagnosed.

“Likewise, a family history of depression is a risk factor,” she continued. “But by using family history alone we would have missed four women who went on to develop signs of depression.”

While these women reported that they also felt more stressed than normal, elevated stress levels, which were based on the women’s answers to the stress questionnaire, weren’t enough to predict which women would ultimately develop postpartum depression. Most of the women in the study reported higher-than-usual levels of stress during the first month after having their babies.

Also, cortisol levels were highest for all of the women in the study at the end of their pregnancies and steadily declined during the month after they gave birth. This ruled out using cortisol as an indicator of difference in stress between women who went on to develop depression and those who did not.

“It was ultimately fatigue that best predicted which women would develop postpartum depression,” Corwin said.

It’s estimated that 12 percent of women develop major depression within a year of giving birth, while about 19 percent develop minor forms of the illness.

Postpartum depression is detrimental on many levels. For one, it interrupts maternal-infant bonding, which may have negative effects on infant behavioral and cognitive development. It can also affect a mother’s self-esteem and her relationship with her partner and other children.

There currently is no standard way to screen for postpartum depression. A mother at risk of developing the illness often goes undiagnosed until several weeks after her baby is born. Treatment typically includes counseling or anti-depressants or a combination of both.

“One of the problems with postpartum depression is that women usually aren’t diagnosed until the disease is already established,” Corwin said. “If a woman’s health care provider knew early on that a patient was slipping down this slope, he or she could intervene. It may not take much to screen for it, either - the questions in the fatigue test that we used took about two to three minutes to answer.”

Corwin conducted the study with colleagues from Pennsylvania State University.