Serena Williams revealed Tuesday that the complications she endured after giving birth were more serious than she originally indicated.

“I almost died after giving birth to my daughter, Olympia,” Williams wrote in a piece for CNN highlighting the challenges women worldwide face while giving birth.

Williams said in a Vogue profile published last month that she was bedridden for six weeks after her daughter was delivered by emergency C-section on Sept 1. She developed blood clots in her lungs and her C-section incision ruptured due to the coughing caused by the clots. When doctors went to close the C-section wound again, they discovered a hematoma in her abdomen. She also had another procedure to insert a filter in a vein to prevent future clots in her lungs.

Williams remained hospitalized for a week, confined to her bed at home for another six weeks and was unable to walk the length of a block until early November. She made her return to competitive tennis in an exhibition match on Dec. 30 and was expected to play in the Australian Open in January, though she was forced to withdraw.