The stress of parenting may set off a relapse for someone in recovery from a substance use disorder, said Katayune Kaeni, a psychologist and board chairwoman of Postpartum Support International.

Karen Bullock, 39, who lives outside Peoria, Ill., had a difficult pregnancy and a traumatic preterm delivery, and she struggled to breastfeed.

“Nothing came naturally,” she said. “I wasn’t happy when the baby was born — I was scared. Every time I looked at him, I would think, I don’t know what to do with you.” Ms. Bullock was eventually diagnosed with postpartum depression and began taking medication.

Complications can surprise even women who had trouble-free pregnancies. After a cesarean section, Aryana Jacobs, a 34-year-old health technology analyst in Washington, D.C., was told that her blood pressure was fluctuating. At home, Ms. Jacobs checked it with a blood pressure cuff she kept because of a family history of hypertension. Within days, the reading had reached 170/110.

She went to the hospital and was treated for pre-eclampsia — which usually develops during pregnancy, not afterward.

“I wish every new mother was sent home with a box of chocolates and a blood pressure cuff to emphasize that you, as a mother, are still a patient,” Ms. Jacobs said. “Your body is recovering from something massive.”