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Sex after childbirth? In the weeks after a newborn’s arrival, it may seem as far-fetched as nightclubbing.

Sexual problems are common among new parents, but discussing them with doctors or close friends is not. And studies of sexuality in the postpartum period have been limited, focusing mostly on the impact of physical changes or the resumption of intercourse. They also have been remarkably one-sided, surveying birth mothers but rarely their partners.

Now new research from the University of Michigan offers a much more nuanced view of relationship dynamics in the so-called fourth trimester — and delivers a few surprises. For one thing, although new parents don’t crow about it on Facebook, plenty of them do steal time for intimacy between diaper changes early on.

In online questionnaires, 114 partners of birth mothers were asked about factors influencing their desire and a range of sexual experiences. A third reported vaginal intercourse by six weeks after birth. Roughly the same percentage of birth mothers had received oral sex by then. Nearly 60 percent of their partners had received oral sex by six weeks.

Sari van Anders, lead author of the study and an assistant professor of psychology, women’s studies and neuroscience at the University of Michigan, cautioned since their sample was not random, “we might be getting more people who participated in our study who are more sexually open.”

Resuming sex after childbirth can be complicated. Sascha Anderson, 30, who works for a Manhattan cheese shop, worried that sex would be painful or that her perpetual dehydration from breast-feeding would strip the act of pleasure. Those first months, she also felt a need to “overassert” her new title as a mom.

“You don’t want to let go of that role as a parent,” she said, adding that she seesawed between two kinds of self-reproach. “You can feel guilty abandoning your child for base desires” or “not engaging with your partner.”

New fathers, too, are in uncharted terrain. The Michigan study found that they may be the ones saying, “Not tonight, I’m exhausted.” The 114 partners, mostly men, reported low desire, primarily because of fatigue and stress.

Ms. Anderson’s husband, Michael, 29, said that stealing a moment to relax was a more urgent priority for both of them than sex. “I don’t know how we’d be able to do our jobs and take care of the baby without a bit of recuperative time,” he said.

In the study, high desire depended on the birth mother’s interest in being sexual and on how close her partner felt toward her.

After their daughter’s birth, when sex was off the table, Andrew Chin, 28, who works in information technology, said cuddling his wife, Dr. Kelly Palchik, a dentist, was their ritual. “Caressing her ears and her neck became really special,” he said. “You want to keep that intimacy.”

Couples often get a green light for penetrative sex at an ob-gyn appointment at four to six weeks. But that checkup assesses vaginal healing, not psychological readiness. Many couples leave the office without realizing there’s a new normal.

“The expectation is we should be returning to the quality of our sex and frequency before,” said Lori Brotto, the director of the University of British Columbia Sexual Health Laboratory in Vancouver. Altering those expectations is key to finding satisfaction again, she and other experts said.

Postnatal sexual problems are common, including vaginal dryness, loss of libido and painful intercourse called dyspareunia. “To say it’s a struggle in a lot of cases is an understatement,” Dr. Brotto said. “In some cases, it’s an outright disaster.”

Enduring sexual difficulties after childbirth can be a taboo topic. One study found that health providers discussed contraception with 96 percent of women, but only 15 percent of those women voluntarily brought up sexual issues.

Dr. Roya L. Rezaee, a co-director of the sexual health program at University Hospitals Case Medical Center in Cleveland, laments that ob-gyns typically discuss postpartum sexual health only when problems arise instead of helping to set expectations earlier and asking women about sexual function. “It rests on us to at least ask an open-ended question, just like we ask if they are incontinent or feeling depressed,” she said. Dr. Rezaee encourages her own patients to take a stepwise approach that doesn’t discount making out. “You can’t go from not doing those things to acts of sex,” she said. “You can’t go from changing a dirty diaper to penetration.”

After the birth of their son, Delondra Williams, 31, a screenwriter in Los Angeles, and her husband Nick, 33, who works for a nonprofit theater organization, had to be patient. She was in pain for months after tearing in labor. She also felt “touched out” with the baby constantly nursing.

For the first three months, “my ‘sexual’ fantasy was for nobody to touch me,” she said. Though his interest had not flagged, Mr. Williams said he was in no rush and a little apprehensive: “It didn’t seem like an option for a while.”

The couple did start to have sporadic sex at three months, although it was often painful for Ms. Williams. Setting some rules helped stoke her desire; one was no touching of her breasts, which had grown to an H-cup to his delight, and her discomfort. They kept trying, and more than a year after delivery, “now sex is great,” she said.

“Women need to know it’s O.K. to be gentle with themselves and be patient,” she said. “If you have open, honest intimacy, it will come back.”