Estrogen and growth factors also surge early in pregnancy. The hormonal changes trigger a cascade of physiological changes. By the fifth week of gestation, the pregnant woman’s cardiovascular system has begun to change. Her total blood volume will increase by 35 to 45 percent over the course of the nine months — as will the total number of red blood cells.
With each heartbeat, she pumps out more blood, and she breathes faster. The changes also affect the kidneys, meaning more frequent bathroom runs.
Seven to eight weeks in, up to 80 percent of pregnant women have nausea that can be quite severe and occurs day and night (“morning sickness” is a misnomer, doctors say).
So how did Ms. Williams do it?
“I’m sure there were moments she had to push through fatigue. But she had a job to do, she didn’t give in, she pulled out all the stops and got the job done,” Dr. Riley said. “A lot of women do that. They have to work.”
“I was nervous,” she said. “I wasn’t sure what to do. Can I play? I had a lot of questions.” She continued: “You hear all these stories about people when they’re pregnant — they get sick, they get really tired, really stressed out.”
“I knew that at that moment, it was important for me to just focus,” she said. “I really felt like I didn’t have time to deal with any extra emotions — any extra anything.” Ms. Williams added: “Pregnant or not, no one knew. Every tournament where I show up, I’m expected to win.”
James Pivarnik, a professor of kinesiology at Michigan State University who has studied the effects of exercise on pregnancy, said research on top athletes is limited, but his hypothesis is that elite athletes are more resilient than most people.
“These women are different. They can recover from incredible stresses that they put themselves through on a daily basis for their entire lives,” Dr. Pivarnik said. “One of the reasons they can hit a tennis ball really hard is that they can recover. It’s the people who can recover who make it to that level.”
Dr. Riley said Ms. Williams’s timing was impeccable because there is no reason to avoid competing during early pregnancy; the heavy lifting of pregnancy starts later, around 20 weeks. (Ms. Williams, who announced her engagement to Alexis Ohanian, a businessman, in December, has not played a match since the Australian Open, and a spokeswoman said she will not return to the circuit until 2018).
Dr. Riley said: “My first thought when I heard was, ‘Oh my goodness, smart woman, your timing was perfect: It’s early in pregnancy and there’s nothing to do but sit tight and wait and keep your fingers crossed all goes well.’ Nothing she did was going to change that — so why not go out and win the Australian Open?”
Dr. Raul Artal, a professor emeritus of obstetrics and women’s health at St. Louis University, said some research has suggested that early pregnancy may even offer an endurance advantage to athletes as a result of the increase in oxygen-carrying red blood cells.
Other athletes who have competed while pregnant include the beach volleyball player Kerri Walsh Jennings, who was five weeks pregnant when she won a gold medal in the 2012 London Olympics. The archers Khatuna Lorig and Cornelia Pfohl were pregnant when they won bronze medals for the Soviet Union and Germany in 1992 and 2000, respectively.
Changes that occur midpregnancy, around 20 weeks’ gestation, may interfere with competitive sports. Dehydration can trigger uterine contractions. Joints become more flexible and lax, increasing the risk of joint strains and injuries. And a woman’s center of gravity changes, affecting balance.
While pregnancy is not the time to start a strenuous exercise program, physicians generally encourage women to remain physically active. Contact sports like ice hockey are discouraged, but the days of advising pregnant women to avoid exercise are gone. Physical activity guidelines are the same for pregnant and nonpregnant women, and those who have been sedentary are encouraged to take up moderate activities like walking.
Regular moderate physical activity can help reduce excessive weight gain during pregnancy and curbs the risk of complications like gestational diabetes and pre-eclampsia.
“Conventional wisdom years and years ago was to tell women, ‘you’re pregnant, stay off your feet, put your feet up,’” Dr. Riley said. But now, unless there are specific reasons not to exercise, she said, “We encourage it, in moderation.”
The 35-year-old Ms. Williams said she plans to return to competitive tennis after her pregnancy.
“I definitely plan on coming back — I’m not done yet,” Ms. Williams said at the Vancouver conference. “This is just a new part of my life. My baby is going to be in the stands, hopefully cheering for me and not crying too much.”