It would be perfectly safe for Williams to continue with rigorous training throughout pregnancy, but how quickly she would return to tournament tennis was uncertain, the researchers said.
Serena revealed her 20 weeks old baby bump in a yellow swimsuit with two gold buttons and a cut out on the side.
Dr. Lauren Streicher, clinical associate professor of obstetrics and gynecology at Northwestern University Feinberg School of Medicine in Chicago, said Williams' age - 35 - increases her chances of pregnancy complications, such as developing high blood pressure.
What is your reaction after learning that Williams was already pregnant when she won her recent title?More news: Volkswagen Going Down EV Route, After Dieselgate Scandal
If her timing was precise, it would mean Williams won the Australian Open, her 23rd Grand Slam crown to set an Open Era record, without dropping a set on January - while eight weeks pregnant.
Williams announced she was engaged to Reddit co-founder Alexis Ohanian in December, after the pair started dating in October 2015.
The tennis superstar will take maternity leave for the remainder of 2017 following news of her pregnancy, according to her rep.
Williams wouldn't be the first top women's player to return to competition after having a baby, but she would be the oldest.More news: Venezuelan opposition to hold 'mother of all marches' against Maduro
Her inactivity saw her drop to second in the world rankings. She will also miss Wimbledon in July and the 2017 US Open holding between August 28 and September 10.
Williams began the first "Serena Slam" in 2002 with wins at the French Open, Wimbledon and US Open and finished the feat at the 2003 Australian Open, defeating Venus in all four finals to join Court, Steffi Graf, Martina Navratilova and Maureen Connolly as the only women to hold all four Slam titles simultaneously.
Commentators on the Tennis Channel on Wednesday, including former star Tracy Austin, noted that other mothers have come back into the sport.
Many people could not believe Williams was able to win a major tournament while pregnant. But lower prize money in tennis left her 40th in the overall standings. With the current No 1, Angelique Kerber, struggling to match her results from 2016, Williams will return to the top spot on Monday.More news: United CEO says no one will be fired for dragging incident