<qa>  What is a genetic vulnerability to depression?- Postpartum depression PPD is a serious syndrome that resembles a major depressive episode and occurs in 10 to 20 of all mothers in the year following delivery. Women with histories of major depressive disorder MDD are at an increased risk for PPD and recurrent PPD with subsequent pregnancies. One possible genetic vulnerability to depression and PPD in particular is the BDNF gene. BDNF is a protein that affects the growth and development of brain cells, including those that help to regulate mood. BDNF levels have been shown to be significantly lower in individuals with depression, including women. Researchers are interested in studying BDNF levels and hormones such as estrogen in pregnant women who have MDD and are at risk for developing PPD. 