If you're a young woman of normal weight you can be pregnant and not know it until just before you give birth, experts say, but it's more likely to happen if you are overweight.
Kate Hudson, of Melbourne, reportedly gave birth about two weeks after she first discovered she was pregnant while on a holiday with friends in Europe.
According to blog The Young Mummy which first reported the story, Hudson was close to 38 weeks pregnant and didn't have a clue.
"I had absolutely no real clear symptoms. I was still getting regular periods. I wasn't nauseous, no morning sickness, little weight gain, so it seemed silly to think I would be pregnant. All I had leading up to it was constipation, and I went from a C-cup to a D-cup," Ms Hudson said.
Earlier this week, we also reported the story of Sydney woman Kim Walsh, who gave birth at Fairfield Hospital just hours after she went to her doctor's surgery suffering abdominal cramps.
Professor Michael Permezel, president of The Royal Australian and New Zealand College of Obstetricians and Gynaecologists, said the hospital he works at sees 6000 births a year. Of those, two or three will involve women who didn't know they were pregnant.
"It's less common in women of normal weight and more common in women who are overweight, but it certainly happens," Professor Permezel said. "It's uncommon without being rare."
Ms Hudson, a media student, reportedly had such a small baby bump that she was was able to board the plane home from Europe without question in August. Women in their third trimester usually need to carry a letter from their doctor or midwife showing they have permission to fly.
Professor Permezel said many women have irregular periods and don't experience morning sickness, so the only sign of pregnancy is abdominal growth.
"If the baby isn't all that big the increase in abdominal girth might not be all that great. Nearly all women do recognise it, but it's not unheard of that a women doesn't pick up a pregnancy," he said.
"Whereas most women are obviously pregnant, there are plenty of pregnant women who could mask the fact, or for whom it wouldn't necessarily be obvious until right at the end."
Professor Andrew Korda, a consultant emeritus in obstetrics and gynaecology at the Royal Prince Alfred Hospital, said that while it was possible for a young woman of average weight to miss the signs of the pregnancy, it is far more common in obese women.
"Most of the people who are in this sort of situation are usually fairly obese so they don't actually see the enlargement of their abdomen," Professor Korda said.
"With obesity comes other problems with menstruation and lot of obese people have infrequent menstruation, so missing a period isn't a problem. Another thing that is always a mystery is how they don't feel foetal movements, but quite often if you're very large the foetal movements can be mistaken for sort of gastrointestinal contractions."
According to The Daily Mail, Hudson said doctors had informed her she had a tilted-back uterus, which causes the baby to grow towards the back, rather than out of the pelvis.
Professor Korda said about 20 per cent of women have retroverted uteruses and while this could hide an initial baby bump, the uterus assumes the typical forward tipped position about 16 weeks into the pregnancy.
Young women could go also go into "psychological denial" about their pregnancies, he said.
"It is possible that sometimes people deny, particularly young girls, the symptoms of pregnancy because they don't want the baby and their emotions aren't capable of coping with it. They are terrified of the consequences and they go into the psychological denial," Professor Korda said.
On the reverse side of things, women can experience a pseudo-pregnancy where they have all the clinical symptoms of pregnancy - weight gain, lactation, morning sickness - without ever being pregnant.
Hudson, who gave birth to baby Mason at the Royal Women's Hospital in Melbourne, said she is starting to process her new life.
"It's hard to get my head around it, even now he's eight weeks old. I wake up in the morning and sometimes forget that I have a baby. It's a huge change and very overwhelming. I didn't have the eight or so months to prepare, so everything has been test and go," she told the blog.
"I honestly don't know if I'm doing it right or wrong. But he's a happy baby and gaining weight, so I must be doing something right."