(CNN) - In January 2017, two months into her pregnancy, tennis superstar Serena Williams competed in -- and won -- the Australian Open. At 20 weeks pregnant, with the smallest of baby bumps, she posted a picture on Snapchat of her athletic body, in a yellow bikini. Hers was a fortunate and healthy pregnancy, by any standard.
However, the days and weeks following the September birth of daughter Alexis Olympia were difficult, according to a recent Vogue magazine interview.
When contractions began, baby Olympia's heartrate plunged. An emergency cesarean section was performed, and Williams' experience with this common surgery -- nearly a third of births in the United States are C-sections -- was flawless.
According to Vogue, Alexis Ohanian, the happy father and co-founder of Reddit, cut the cord, and Olympia was laid on her mother's chest.
"And then everything went bad," Williams, 36, told Vogue in an interview confirmed by her publicist.
Despite her obvious strength and good health, the world champion tennis player has a history of blood clots. Clots that form within veins or arteries disrupt the circulatory system and can damage the organs. Smoking, overweight and obesity, and genetics are all factors that may cause blood clotting.
Williams takes blood thinners every day to prevent clots from forming. After the C-section, though, she stopped taking them to allow the surgical wound to heal.
The next day, off the medication, the 23-time Grand Slam winner began to gasp as she recovered in her hospital room.
Not wanting to worry her visiting mother, Williams stepped into the hall and flagged a nearby nurse, insisting that she needed an IV with heparin, a blood thinner, and a CT scan to check for clots.
The nurse believed that medications might have befuddled Williams, Vogue says, but a doctor arrived -- only to perform an ultrasound, not the requested scan.
The ultrasound revealed nothing, and according to Vogue, Williams reiterated: "I told you, I need a CT scan and a heparin drip." Obeying her request for the scan, the medical team found several small blood clots in her lungs and immediately began the medication.
Williams' physical instincts were correct. Still, a six-day medical crisis followed.
First, her C-section wound reopened after a coughing spell caused by the clots in her lungs.
Next, she returned to surgery to correct the lung clot, called a pulmonary embolism, and her medical team found a large hematoma, or clotted blood in the tissue, in her abdomen. The cause: the blood thinner she'd resumed.
Another operation allowed a surgeon to insert a filter into a major vein to prevent more clots from dislodging and traveling to Williams' lungs.
Finally, a week after the birth, she returned to her Florida home. Sadly, the new mother found no comfort in her daughter's cries.
The medical ordeal combined with new motherhood temporarily overwhelmed Williams. She spent six weeks in bed.
"I was happy to change diapers," Ohanian, who married Williams in November, told Vogue, adding that the hardest part was "not being able to help."
Williams told the magazine she felt depressed sometimes.
"No one talks about the low moments," Williams said. "The incredible letdown every time you hear the baby cry. Or I'll get angry about the crying, then sad about being angry, and then guilty, like, 'Why do I feel so sad when I have a beautiful baby?' The emotions are insane."
Today, Williams is raising her daughter and has made a return to the tennis court.
On December 30, she lost an exhibition match against Jelena Ostapenko, the 2017 French Open champion, at the World Tennis Championship in Abu Dhabi.
Based on this uncharacteristic result, Williams pulled out of the Australian Open, which starts Monday, and continues to train. Her schedule did not permit time for an interview with CNN.
Williams' sights remain set on the record of 24 Grand Slam titles held by Margaret Court -- though she won't push baby Olympia to follow her path. "I'm so glad I had a daughter. I want to teach her that there are no limits."
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