‘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,’ she told the magazine.

Gynecologists have rushed to clarify that it is not possible to have a menstrual cycle while pregnant, because a period is the shedding of uterus lining that is not needed for pregnancy.

However, it is very common for women to experience vaginal bleeding that seems like a period, but is actually the result of something else.

In fact, female athletes are particularly at-risk for surprise pregnancies because regular heavy exercise has been shown to affect the regularity of ovulation.

Dr Alyssa Dweck, a New York OBGYN, Dr Lauren Streicher, a professor of obstetrics and gynecology at Northwestern University, and Aparna Sridhar, a UCLA gynecologist, explained to DailyMail.com the various reasons for vaginal bleeding in pregnant women.

Williams insists she was as surprised as her fans, having not seen her now-husband Alexis Ohanian in a month – and she only took the pregnancy test to ‘shut up [her] friend’ who was suspicious

YOU CAN BLEED WHILE PREGNANT – BUT YOU CANNOT HAVE A PERIOD

If a woman misses her period it is a sign of pregnancy.

The reason being that menstruation only happens in women of a reproductive age who are not pregnant.

During each menstrual cycle, the uterus builds up a lining of blood, ready for an embryo to implant itself and start growing.

If an embryo does implant, the blood lining stays intact. That is because, once the egg implants, hormones send signals to the tissue in the uterus, telling it that cushion of blood is necessary for the pregnancy.

If no eggs are fertilized, the tissue and blood shed.

MOST LIKELY CAUSE: IMPLANTATION BLEEDING

‘This happens all the time,’ Dr Dweck told DailyMail.com, when she heard of Williams’ confusion about bleeding while pregnant.

Dr Sridhar concurred. ‘About 20 to 40 percent of pregnant women can bleed before the 14th week (in the first trimester),’ she said.

According to Dr Streicher, there is one most likely cause: ‘It wasn’t a period, it was bleeding. Possibly some implantation bleeding,’ she said.

About a third of women experience this about six to 12 days after conception.

After the sperm fertilizes the egg and becomes an embryo, the embryo moves down to the uterus to implant itself into the uterus wall.

In some cases, this implantation can be imperfect, and causes a little bit of blood to shed down through the vagina.

Usually, this implantation error only triggers light bleeding – much lighter than a regular menstrual cycle – and anything heavy requires medical attention.

The tennis champion, 36, gave birth to her daughter with Alexis Ohanian, Alexis Olympia (pictured), in September 2017

OTHER CAUSES

Beyond implantation of the pregnancy, Dr Sridhar said, ‘miscarriage or threatened miscarriage are the common causes. There can also be bleeding from the cervix, vagina or uterus.’

This may be triggered by an infection or inflammation, polyps, or abnormal cervical cancer cells.

Ectopic pregnancy is another possible cause Dr Sridhar would explore if her patient experienced bleeding while pregnant.

An ectopic pregnancy involves an egg which fertilizes outside the uterus. The egg cannot survive, but if it is left to grow it may damage organs, causing bloodshed.

Smoking, hormonal imbalances, and infections increase the risk.

Lastly, a mother’s age can drive up the risk of gestational bleeding, if she is over 40, for example.

Williams was competing in the Australian Open when she found out she was pregnant. She went on to win the tournament despite being two months along (pictured competing in the final on January 28, 2017)

FEMALE ATHLETES OFTEN HAVE IRREGULAR PERIODS – WHICH CAN MAKE PREGNANCY UNPREDICTABLE

Few women have a perfectly regular period that comes at the exact same time of the month every month, bleeding the exact same amount, and lasting the exact same number of days.

Most women experience some sort of irregularity due to their fluctuating levels of estrogen, progesterone and testosterone.

It is crucial to have those hormones working in synchronization to keep the cycle ticking. Ovulation is incredibly complex, and requires sufficient amounts of each hormone for each step of the process.

When those hormones are fluctuating, and not working in synchronization, it can mean a woman does not ovulate for months, or ovulates on a seemingly random schedule.

It can cause ovarian follicles to burst, causing light bleeding, or an increase in estrogen levels, which can also cause light bleeding.

Often, this irregularity is mild, but in some women the fluctuation of hormones is particularly volatile.

‘You can just bleed at any time during your cycle and it seems like a period but it’s just bleeding,’ Dr Dweck said.

‘That’s why, when people ask if they can have 𝑠e𝑥 without getting pregnant on their period, I say to be careful. You cannot get pregnant on your period, but unless you have a perfectly normal cycle, you can’t be sure. It may not be an actual period.’

Who is most at-risk?

Female athletes often experience this kind irregularity, which studies have attributed to vigorous exercise derailing the balance of hormones.

Other causes of irregular ovulation include PCOS (polycystic ovary syndrome), a disease which slows the development of ovarian follicles, and increasing age.