"I nearly died after giving birth to my daughter, Olympia", Williams - who made her competitive return in the Fed Cup but lost a dead rubber alongside sister Venus - wrote in her column for CNN.
Tennis champion and legend, Serena Williams gave birth all the way back in September but has been outlining just how tough her road to recovery has been so far. But they discovered a hematoma in her abdomen, further complicating the damage.
"First my C-section wound popped open due to the intense coughing I endured as a result of the embolism", Williams wrote. I returned for a procedure that prevents clots from travelling to my lungs.
"If it weren't for [my doctors'] professional care, I wouldn't be here today", she wrote. In 2011, she spent almost 12 months incapacitated after a cut on her foot from a piece of broken glass at a Munich restaurant led to a pulmonary embolism.
Williams, thanked the medical team who looked after her, adding that she owed them her life. She went on to urge fans to demand change and donate to UNICEF. "Yet I consider myself fortunate".
"When they have complications like mine, there are often no drugs, health facilities or doctors to save them", she wrote. "That same day, approximately 2,600 babies died on their first day of life".
UNICEF says 80 percent of the almost 2.6 million newborn deaths around the world each year are due to preventable causes, Williams notes. Over 80% die from preventable causes.
Making a wider point about mothers' health, Serena cites statistics that black women in the U.S. face higher birth mortality rates than their white counterparts, and that death during childbirth is a too common occurrence in poorer countries.
Now, Williams wants to help women with like conditions.