COLUMBUS, Ohio (WPVI) --Doctors say they have new clues into what causes depression during pregnancy.
It's pretty common: About 1 in 7 women experience depression before delivery, and half a million women have postpartum depression.
Doctors at Ohio State University Medical Center have linked it to a protein in the brain.
The protein known as BDNF (brain-derived neurotrophic factor) dips during pregnancy.
BDNF helps to regulate our moods.
But during pregnancy, it's also vital for the placenta and a baby's brain development.
In testing the blood of 139 women during and after pregnancy, researchers noticed it drops between the first and third trimesters, but women with the lowest levels during the second and third trimesters are more likely to have depression.
They also found that women with significantly lower BDNF levels in the third trimester had a higher rate of low birth-weight babies.
Normally, levels of the protein rise again after birth.
Anti-depressants and exercise both boost BDNF, so women at high risk of depression have several options.