When you're a breastfeeding mama it can feel like your breasts fluctuate more than interest rates. When one's up, the other's down and the bra you put on in the morning might not fit by the afternoon.

Tennis superstar Serena Williams knows that feeling all too well. The new mom welcomed daughter Olympia with husband Alexis Ohanian in September, and in a new interview for the August edition of InStyle, she dropped some truth about how much a lactating mother's breasts can change throughout the day.

"I'm in the locker room pumping before a match because my boobs are so big," she says. "When I pump, they go down a size or two and I go out and play."

This isn't the first time Williams has talked candidly about the reality of balancing breastfeeding, and motherhood in general, with her physically demanding career.

It hasn't been an easy task. Like so many mothers returning to work after giving birth, Williams has been penalized for taking the leave she needed , and in a previous interview with Harper's Bazaar , Williams said she knew she would need to quit breastfeeding before the upcoming competition at Wimbledon next month.

While Williams has been able to balance breastfeeding and training for most of Olympia's life, it hasn't been easy, and her coach Patrick Mouratoglou advised her that she couldn't keep doing both if she wanted to win at Wimbledon this year, as any extra weight would put her at a disadvantage on the court. In an episode of her HBO show, Being Serena , viewers saw Mouratoglou tell Williams point blank, "There is no miracle. You stop breastfeeding."

Speaking to Ohanian in another scene, Williams describes the conflicted feelings so many mothers face when deciding how and when to stop breastfeeding. "My whole fear is that I'm going to hold her and then she's going to turn to me, and I'm not going to have any milk," she said. "It's going to literally break my heart."

Now that Olympia is older and eating solids (berries and beets are a a fave according to her mama's Instagram) the breastfeeding dilemma isn't such a huge issue anymore. But Williams still says feeding and snuggling with Olympia is "the best part of [her] day", so figuring out how to balance tennis training time with parenting time is a top priority.

"The only rule I have at practice is to be done at 1 p.m., because as much as I love tennis, I need to be with her. I want to put her above everything else I'm doing," she tells InStyle .

And just like we can relate to the ups and downs of bra sizes, we can relate to that.