Although in some cases, dark thoughts can be a severe and debilitating symptom of a postpartum anxiety or depressive disorder, usually they are just an unpleasant part of normal adjustment.

CNN: When motherhood becomes a nightmare

New mothers may also be reluctant in bed because of physical discomfort or because they have some element of postpartum depression, said Dr. Rini Ratan, assistant clinical professor of obstetrics and gynecology at Columbia University Medical Center.

CNN: New moms and the post-baby sex slump

Cognitive behavioral therapy -- which focuses on changing thought patterns -- and antidepressant or anti-anxiety medications are all treatment options for women with postpartum depression, Kleiman said.

CNN: Postpartum depression: When moms feel out of control