In an interview with InStyle the tennis superstar – who found out she was seven weeks along while competing in the 2016 Australian Open – says she only took a test to “shut up [her] friend” who was suspicious.
“I literally had a cycle just before. So I was surprised when I saw the result and even more surprised when the doctor said I was seven weeks along,” the 36-year-old said.
While experts have clarified that what Serena experienced was not actually menstruation, light bleeding during early pregnancy happens to around a quarter of mums-to-be.
"That’s why when pregnant women have a dating scan, it may be different to a pregnancy test result, as a implantation bleed could be misinterpreted as a period," Dr Sonya Jessup, fertility expert at Demeter Fertility, told Women's Health.
Bleeding during the early stages of pregnancy is not normal, but not uncommon Sonya explains, and the usual culprit is the implantation of the egg in the lining of the uterus. However it is still important to speak to a doctor to ensure that you and your baby are healthy.
"It's always important to contact your doctor if you have any bleeding during pregnancy," Sonya says. "If a woman has a rhesus negative blood type she may require an injection of anti-D if she is over six weeks pregnant to avoid complications in future pregnancies."
"The awareness of a possible implantation bleed is particularly important for women undergoing an IVF cycle, as they may be tempted to stop medication early thinking their period has arrived, when in fact it’s extremely important that they continue their medication until their next pregnancy test is due, otherwise they can increase the chance of miscarriage."
Bleeding in the later trimesters of pregnancy is less common and can be the sign of a serious problem like placenta previa (separation of the placenta) or placental abruption (when the placenta covers the cervix). It is essential you speak to a medical professional if this happens to you.