When one of my children was a baby — maybe the first one — I remember someone offering a cheerful piece of workout advice: You start by lifting this 8-pound-or-so weight, and then little by little, in infinitesimal daily increases, you’ll work up to lifting a 10-pound, 15-pound, 20-pound weight — and that’s parenthood for you! A terrific workout — you’ll both be in great shape by the time you have a 1-year-old.

But those early months as that baby grows and gains weight can be pretty hard on a parent’s muscles, because none of that “weight lifting” is actually guided by the strengthening — or the safety — of the parent. And mothers in the postpartum stage are especially vulnerable to injury, because hormonal changes in pregnancy have left the ligaments and joints looser.

Certainly, “repetitive stress” (like “terrific workout”) sounds like a metaphor for parenting and for family life in general, but it’s decidedly only a partial metaphor. Still you can very literally get injured, and physical therapists do warn that it’s important to be especially careful during those months of lifting, dressing, diapering, carrying and car-seating your ever-imperceptibly heavier bundle of joy.

A recent survival guide for the fourth trimester, that is, for the postpartum body after the rigors of pregnancy and childbirth, emphasized the importance of core strength. The postpartum body is different, and differently vulnerable, and it’s not just a matter of residual extra pregnancy weight. “You’re not the same as an overweight woman working out,” said Dr. Karen Sutton, an orthopedic surgeon and sports medicine specialist at the Hospital for Special Surgery in Stamford, Conn. “Your body is completely changed.”

Dr. Sutton, who has four children, said that she used to think, “I just have to lose the weight, that’s all that needs to happen.” But paying attention to the realities of postpartum women — including surgeons — has taught her that there are important differences, which affect exercise and injury. The ligaments are laxer, she said, and “everything’s a little looser.”

During pregnancy, as a woman’s abdominal muscles are stretched, the rectus abdominis begins to pull apart, causing a separation — a diastasis — between the two sides of the abdominal muscle. “You have basically no core after giving birth,” Dr. Sutton said.

[Try these prenatal exercises recommended by the Hospital for Special Surgery.]

That diastasis decreases your core stability, said Anna Ribaudo, a physical therapist at the Hospital for Special Surgery who sees many postpartum patients, and “decreasing stability may put more pressure on someone’s back or someone’s hip or someone’s knee.”

Aggressively working out too soon can actually make everything worse, Dr. Sutton said, but even the basic lifting and carrying activities around the house with the new baby can be problematic. Women need to adjust to lifting less weight than they were perhaps previously accustomed to, right at the same moment they may be carrying baby strollers up the steps.

The muscles of the pelvic floor, Dr. Sutton said in an email, can be a key to success in helping your body function well, and eventually for getting back to exercise. “Without full engagement of the pelvic floor muscles, back pain may ensue, the hamstrings will not be fully engaged, your abs will have trouble making a comeback and even breathing can be an issue.” Weak pelvic floor muscles, she said, can lead to poor posture and to back pain.

Dr. Ribaudo said that some women think that once they have been “cleared” at their six-week checkups, they can go back to their pre-pregnancy levels of activity. “Your body has gone through trauma,” she said, and it needs more time.

And yet, of course, those postpartum months involve constant lifting and bending, for fathers as well as mothers. “I see a lot of postpartum upper back and shoulder and neck pain,” she said, because new parents tend to be bent over so often. She advises her patients to think about the setting in which they will be changing or dressing or bathing the baby, and try to raise the height so it involves less bending.

“Every single time you change a diaper or dress the baby,” she said, “stop and do 10 scapular retractions” — pulling the shoulder blades toward the spine — “just to undo what you did being bent over.”

And then there’s feeding: “You’re going to be spending so much time feeding your child, whether breast or bottle,” she said, and you shouldn’t actually be holding up the baby all that time. “Your arm should be supported so you can relax those muscles and not be constantly straining them.” Even for something as simple as how you hold a bottle, she said, make sure to keep your wrist as neutral as possible, rather than fully flexed.

“As physical therapists, we educate patients that the combination of bending, lifting and twisting is a good recipe to throw out your back,” Dr. Ribaudo said. So what do you do about getting the child in and out of a car seat? “I teach them, take your time, face the car seat, bend at the hips and knees, don’t twist,” she said. “I teach them how to engage their core muscles, give themselves a little more stability.”

Taking even five to 10 extra seconds to think about what you are doing and how you are doing it can make a big difference, she said, especially for parents who feel rushed. Be aware of what you are asking your body to do, walking up a flight of stairs carrying a baby, and perhaps a stroller. Choose lighter strollers when possible if you’re going to be carrying them, and be careful about using baby carriers for long distances once the child gets heavier.

And don’t underestimate the importance of fatigue, she said, and its impact on your posture. “A slumped posture puts more tension on shoulders, back and neck,” she said.

With all that, many new mothers are eager to be active and to build their strength back. Once a woman feels comfortable and has checked with her obstetrician, Dr. Sutton suggested starting with some easy exercises like the plank pose, the bridge pose, the cow pose paired with the cat pose, wall squats and, of course, the pelvic floor exercises known as kegels.

“It took your body 10 months to get here,” Dr. Ribaudo said, and it might take roughly as long to get back to where you were before pregnancy. “I think unfortunately in our society, giving birth has become something where everyone’s expected to bounce back in a month or two, and it’s just not true.”