One of the contributing factors in the U.S. baby-formula shortage is a significant shift in the way parents feed their babies: Breast-feeding declined during the pandemic, reversing a decadeslong trend, health practitioners say.

Since 2020, the share of breast-fed one-year-olds has plummeted from an estimated 34% to an estimated 14% this year, according to surveys conducted by Demographic Intelligence, a forecasting firm that specializes in births and works with formula manufacturers including Abbott Laboratories and Nestlé SA . Because of the small sample size, the firm’s 2022 estimate has a range of error of plus or minus 6 percentage points.

Government officials and formula manufacturers are working to address a nationwide shortage that in recent months has resulted in empty shelves at some stores. Causes for the shortage include pandemic-related supply-chain snarls, a February recall by Abbott, as well as the closing of one of its factories.

An increase in demand for formula, driven by the decline in breast-feeding, has exacerbated the problem, according to Lyman Stone, director of research for Demographic Intelligence.

After Covid-19 restrictions were introduced in March 2020, many new mothers had shorter hospital stays and were discharged before their milk had come in or their baby had latched successfully to their breast, breast-feeding experts say. Some infants weren’t given skin-to-skin contact with their mothers after birth because of concerns about Covid-19 transmission.

Some lactation consultants were furloughed, redeployed or designated nonessential personnel; others offered only virtual appointments. Parents had less access to in-person assistance from doulas and peer-support groups. They also had less help from family and friends, who stayed away to avoid exposing newborns to the coronavirus.

Parents who want to breast-feed need a strong support network, said Diane Spatz, a professor of perinatal nursing at the University of Pennsylvania and a nurse scientist at Children’s Hospital of Philadelphia. In the first few days after birth—a critical period for establishing a mother’s milk supply—many families need help troubleshooting. And more broadly, if a mother spends time with other breast-feeding parents, she is likely to nurse her child longer.

“It takes a village,” Dr. Spatz said. During the pandemic, she said, “all the in-person, peer-to-peer support went away.”

Ms. Snyder feeds Gavin in their home in Platte Woods, Mo.

Photo: Katie Currid for The Wall Street Journal

Nicole Snyder’s son, Gavin, was born prematurely last September in Kansas City, Mo. For two weeks while he was in the neonatal intensive-care unit, she tried to pump breast milk every two hours. Lactation consultants were on hand to help. But her mother-in-law, who is a nurse, was barred from visiting her in the hospital, along with her mother and other family members. That heightened Ms. Snyder’s feelings of stress and isolation, which she said contributed to her inability to produce more than a few ounces of milk a day. She and her husband now feed Gavin an Enfamil-brand formula.

“If I could have had visitors, I might have had a little more sanity,” said Ms. Snyder, 34, who works as a middle-school art teacher. “I had a lot of things working against me.”

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The American Academy of Pediatrics recommends breast-feeding exclusively for the baby’s first six months, then continuing to breast-feed while introducing complementary foods until the child is at 12 months or older.

Some parents can’t breast-feed their children; others choose not to. Infant formula is the only safe alternative to human breast milk for babies under a year old, health officials say.

Breast-feeding rates in the U.S. reached a low point in the 1970s, when many doctors told parents that formula was the best food for babies. Then a movement to promote breast-feeding, and growing research showing the benefits of breast-feeding over formula, led to a decadeslong increase in breast-feeding.

The share of one-year-olds who are fed with at least some breast milk climbed from 16% in 2001 to 36% in 2017, then plateaued in 2018 and 2019, according to latest data from the Centers for Disease Control and Prevention.

Ms. Snyder shows photos of herself and Gavin after he was born.

Photo: Katie Currid for The Wall Street Journal

Ms. Snyder plays with Gavin in their home.

Photo: Katie Currid for The Wall Street Journal

The recent drop in breast-feeding has been particularly steep among lower-income families and people of color, Dr. Spatz said. At Children’s Hospital of Philadelphia’s largest pediatric primary-care center, which serves those populations, the percentage of breast-fed babies fell sharply in 2020, Dr. Spatz said. Since then, the share of breast-fed babies at the center has risen but hasn’t returned to prepandemic levels, she said.

The pandemic wasn’t a barrier to breast-feeding for all families. For some, it offered advantages, allowing mothers to nurse for longer because they were working from home, said Kim Hekimian, an assistant professor of nutrition at Columbia University Irving Medical Center.

Jessica Hernandez says she stopped breast-feeding her son two weeks after the pandemic lockdown began.

Photo: Ruyman Hernandez

Other families found workarounds to pandemic restrictions, talking with experienced relatives on video calls, scheduling virtual appointments with lactation consultants and calling support-group help lines.

And the expansion of insurance coverage for telehealth visits might have made lactation consultants accessible to more families than before the pandemic, Dr. Hekimian said.

Still, the pandemic placed heavy burdens on parents.

Jessica Hernandez, who lives in rural Washington state, stopped breast-feeding two weeks after the pandemic lockdown began in March 2020. She didn’t have the time to sit still for 40-minute stretches nursing her then-five-month-old baby Teo while trying to home-school a four-year-old and a seven-year-old.

“There was no way that I could keep up with everybody,” Ms. Hernandez said. Once she switched to formula, she said, she could hand Teo to her oldest daughter and say, “OK, feed your brother.”

Write to Jennifer Maloney at jennifer.maloney@wsj.com