Serena Williams has never liked the word “retirement”. Her move away from tennis, announced in an essay in the September issue of Vogue, is an “evolution”, she says. In her transition, she will shift focus from tennis to “other things” that are important to her. One is her wish to have another child.

Williams and her husband have been trying for a baby in the past year, a move apparently encouraged by their four-year-old daughter, who has hopes of becoming a big sister. But, as Williams told the magazine: “I definitely don’t want to be pregnant again as an athlete. I need to be two feet into tennis or two feet out.”

Williams was two months pregnant when she won the Australian Open in 2017. She gave birth to her daughter, Alexis Olympia Ohanian Jr, on 1 September that year. It was not a straightforward pregnancy, however. Williams had a caesarean section after developing a blood clot in her lung during labour, and went on to play through postnatal depression.

There is no doubt pregnancy can take a toll on elite athletes. The weight gain and change in body shape affect balance and posture, which take some adjusting to, and training at maximum intensity should be avoided, says Prof Kari Bø at the Norwegian School of Sport Sciences in Oslo. Any training that carries a risk of the bump being hit, either through impact or a fall, is also strongly discouraged.

But while training tends to be scaled back, athletes do continue to exercise through pregnancy. How soon elite female athletes return after giving birth depends on how smoothly it goes. Beyond any muscle weakness from disrupted training, the two muscles that run down the middle of the stomach often separate in pregnancy as the expanding womb drives them apart.

At the same time, ligaments and other connective tissue loosen to make it easier for the child to come out. The pelvic floor muscles can become stretched and weakened, leading to urinary and faecal incontinence. All can have an impact on recovery.

Further issues can occur after birth. When a woman is breastfeeding, her oestrogen levels drop. This reduces the body’s ability to absorb calcium, which in turn drives a loss of bone density. The risk is that an elite athlete returns to high-intensity training or competitions too soon and fractures a bone. “It’s an issue we have to take into consideration, especially for endurance athletes. They have to have a balance with nutrition and exercise,” says Bø.

Not all of the changes are negative, though. In pregnancy, the heart reshapes and can pump more blood around the body, but the changes are not long-lived, reverting to normal within a couple of months of childbirth.

Despite the challenges, there is no shortage of women who have returned to elite tennis. Margaret Court gave birth to her first child in 1972 and won the Australian Open, the French Open and the US Open in 1973. Evonne Goolagong Cawley and Kim Clijsters also took titles in motherhood. “If everything goes smoothly and there are no complications, it is possible to get back into shape and to improve on past performance, it doesn’t need to be a setback at all,” says Bø.

Candice Lingam-Willgoss, a senior lecturer in sport and fitness at the Open University, says one of the biggest problems for elite athletes in motherhood is the loss of recovery time. Sleepless nights can make it harder to train at full intensity, but the time previously set aside for crucial recovery is so easily taken up with childcare.

The most striking truth in Williams’s essay is that if she was a man, she would not be in this position. “I’d be out there playing and winning while my wife was doing the physical labour of expanding our family,” she writes. Female fertility drops from the age of 35, with the chances of conceiving at 40 about 40-50% in a year. In June, when 36-year-old Rafael Nadal announced that his wife was pregnant, he noted: “I don’t think it will change my professional life.”

Says Lingam-Willgoss: “A lot of elite athlete mothers are still struggling with cultural norms that see women as the caregivers. Motherhood is very selfless and being an elite athlete is very selfish and you are in this constant tension of trying to do both things very well.”

One of the toughest challenges for Williams – and for any elite athlete – is the fundamental loss of identity that comes with leaving the sport. But here Williams has the advantage. “Saying goodbye to that athletic self is very, very difficult. It’s everything she has worked for, it’s who she is,” says Lingam-Willgoss. “But she has already got another identity, and she is thinking of becoming a mother again, and that can mitigate some of the psychological impact. She has already got that new focus.”