Rhiannon White thought she understood her body. She was in her late thirties, educated, working in tech, living in the United States. And yet, when she and her husband started trying for a baby, she realized how little she actually knew about the menstrual cycle—the biological system she had lived in, month after month, for decades.

“I was shocked,” she says. “Frankly, I was ashamed to realize what I didn’t know.” White didn’t know, for instance, that the luteal phase needs to be a certain length to support a pregnancy. Or that implantation happens over several days. Or that, hormonally speaking, “you can be a little bit pregnant,” as your body responds to a possible pregnancy before implantation has fully taken place.

“I’m endlessly appalled and upset by how much we don’t know about this biology that we live in every day,” she says.