text: The 'greatest female tennis player of the Open era' won her 23rd Grand Slam without dropping a set.
  But when you learn she did it while in the early stages of pregnancy, the feat becomes exceptional.
  So how is it possible to win a Grand Slam while pregnant?
  Dr Markos Klonizakis, a senior research fellow at Sheffield Hallam University, says the triumph at that stage of pregnancy is "amazing".
  "It is not easy for any woman to adapt to changes in her body, let alone while playing sport at an elite level," he said.
  "Physiologically, the main challenge women face within about five weeks of pregnancy is in adapting to changes to the cardiovascular system.
  "These are rapid and ensure blood and oxygen supply to the foetus.
  "Many women feel they cannot breathe as easily as their heart rate increases.
  "The nature of a Grand Slam tournament, where players have to recover to play consecutive matches, would have been a challenge for her, if you take into account nausea as well."
  Professor Janice Rymer, of the Royal College of Obstetricians and Gynaecologists, added: "For elite athletes, a tailored training and nutrition plan would normally be developed with a specialist team.
  "High levels of exercise at around eight weeks gestation should not affect pregnancy for these athletes and those used to high levels of exercise.
  "During the first few weeks of pregnancy these hormones may actually boost physical performance as a woman's natural production of steroids will increase slightly."
  Williams is not the first elite athlete to compete while pregnant.
  British Olympic cycling champion Laura Kenny told BBC Radio 5 live: "I was still competing when I first found out I was pregnant. I actually won the madison nationals with Elinor Barker when I was about five or six weeks pregnant, but any time after that I just feel like it is so intense that I wouldn't have been able to [compete]."
  Nigerian table-tennis player Olufunke Oshonaike who appeared at her sixth Olympic Games in Rio - only the second African women to do so - carried on playing when she was seven months pregnant, despite her "big belly".
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  Only last week, American swimmer Dana Vollmer competed in an elite 50m freestyle race while six months pregnant.
  "As hard as people think this is, the race is only 30 seconds long as opposed to the entire day I spend holding and chasing around a 35-pound two-year-old," she said.
  "This will feel like a break."
  After winning gold in the 100m butterfly in the 2012 London Olympics, Vollmer took time off to have her first child, son Arlen, and returned in time to qualify for Rio.
  But this time around, she has made the decision to continue training. Baby number two, another boy, is due in July.
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  "Putting the health of the baby first doesn't just mean sitting on the couch," the 29-year-old said.
  In June 2014, Alysia Montano competed in the 800m quarter-finals of the US track and field championships while eight months pregnant.
  The then 28-year-old runner, who received a standing ovation after completing the race in 2 minutes 32.13 seconds, told the Daily Mail: "I've been running throughout my pregnancy and I felt really, really good during the whole process."
  Her finishing time was 35 seconds slower than her personal best of 1:57.34, but she added: "I just didn't want to get lapped and be the first person to get lapped in the 800m."
  Five-time Olympian and mother-of-two Jo Pavey told BBC Sport: "It is difficult for sportswomen because [Williams] might not have known she was pregnant.
  "I chose not to compete when I was pregnant. I did run round a women's 10k just to keep fit, but I didn't run as far as I could.
  "I chose not to push myself to the limit, just to keep fit and active."
  And marathon world record holder Paula Radcliffe said in 2015: "My priorities changed the minute I knew I was pregnant, and everything I did centred around the baby.
  "I lost that competitive instinct. It wasn't about running certain times in training anymore."
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  Williams' incredible feat led to a bout of introspection on social media, captured by BBC Sport's Sportsday Live debate under the heading:
  'Serena Williams won the Australian Open when she was pregnant, but...'
  Here's the best of your answers:
  Danny Kibbey: So Serena won a Grand Slam at 8 weeks? Pff, my missus completed IRONMAN WALES at 12 weeks (I watched on telly).
  Mike T: Serena Williams was pregnant when she won the Australian Open and I can't even be bothered to finish this senten...
  Taryn Finley: Serena Williams was pregnant when she won the Australia Open in Jan, but I can't even get out of bed when I'm on my period.
  Raun Anand: Serena Williams won a Grand Slam whilst pregnant and I have trouble reaching for the remote after a McSpicy.
  Alison Hennessey: She won the Australian Open while eight weeks pregnant. And I complain about a dynamic yoga class..
  Lisa: Serena Williams was pregnant when she won the Australian Open and I struggle to walk upstairs after a big lunch.
  Aquelious: Serena Williams won the Aus Open when she was pregnant but I get tired if my FIFA17 match goes to extra time!!!
  Chimp: Serena Williams won the Aus Open when she was pregnant, but I once completed a 24 hour Le Mans race on Gran Turismo. AND won.
  Jablesfifa: Serena Williams won Aus Open while pregnant, but I got subbed off in the first half of a football match due to a wasp sting.
target: With or without the benefit of hindsight, Serena Williams' victory at the Australian Open in January was sublime.