In an interview with InStyle magazine, tennis champion Serena Williams said that, during her recent pregnancy, she was convinced she wasn't expecting as she'd just had her period.
She told the magazine she only took a pregnancy test to "shut up" a friend of hers who was suspicious she might be expecting.
"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 says bleeding in early pregnancy is common, affecting one in four women. The amount a woman can bleed is variable, as is how long it can go for. Some women may bleed as a one-off event, while for others the bleeding can be ongoing.
If the bleeding goes for a few days, and occurs around the time you usually get your period, it's easy to see why women can be convinced it's simply that time of month.
The truth is, you can't actually have your period when you're pregnant (your period is your body's way of shedding the lining of your womb because there's no baby on the way).
But there are reasons why you might think you're having one. These include:
In the early stages of pregnancy, the embryo needs to attach and invade into the mother's womb, and the placenta needs to form. This process is known as implantation and can lead to bleeding.
Because this kind of bleed commonly lasts for a few days and then stops, it can easily be mistaken for a period.
Bleeding from the cervix
The cervix is the opening of the womb (known as the uterus) and has two different types of tissue lining it in different parts. The place where those tissues meet is known as the 'transformation zone'.
The transformation zone on the cervix can become "fragile" in pregnancy because of high oestrogen levels, says Dr Lew.
When it's fragile, it can bleed either spontaneously, or after contact (such as after sexual intercourse), making you feel like it's that time of month.
Subchorionic haemorrhage or haematoma
Sometimes you can bleed, or get a blood clot, under the placenta. If some of that blood then escapes vaginally, you may think there's no way you could be pregnant.
(Dr Lew reassures that most of these clots or bleeds resolve over time.)
What should you do if you get 'period-like' bleeding when you're pregnant?
If you know you're expecting and start bleeding, Dr Lew advises seeing your doctor. She says this is especially important if you have a Rhesus negative blood group as you may need anti-D therapy, or if you're in pain or have other symptoms worrying you.
But if you're just experiencing mild bleeding in pregnancy and you've seen your doctor, don't panic.
It's worth avoiding sex, and using pads rather than tampons. Some health care professionals may also advise taking it easy and resting.
If you need pain relief, steer clear of non-steroidal drugs such as ibuprofen, and stick to paracetamol instead.
But if, like Serena, you're convinced you can't be pregnant because you just had your period, think twice.
As the new mum of Alexis Olympia Ohanian Jr. just proved, it is possible to have what you think is your 'period' when you're already on your way to becoming a mama…