One in 10,000 - that was the chance of Ohio identical twins Jillian and Jenna, born last week, sharing the same amniotic sac and placenta.
Here are ten of the more unusual mothers and babies from around the world - and the numbers that make them so rare.
1. 69 - the number of children born from one woman
Feodor Vassilyev gave birth to 69 children between 1725 and 1765 in Russia: 16 pairs of twins, seven sets of triplets and four sets of quadruplets.
2. 2 billion - the number of mothers in the world
3. 375 days - the longest recorded pregnancy
While most pregnancies last around 280 days, 25-year-old Beulah Hunter gave birth after 375 days. Her daughter, Penny Diana was healthy and normal at birth.
4. 10.8kg - the heaviest baby born to a mother
In 1879, the second child of Canadian Anna Bates, who stood some 2.27m tall, was a boy who weighed 10.8kg and was 71 cm long when he was born. After the difficult birth, he only survived for 11 hours.
5. 260g - the lowest weight recorded for a surviving baby
The smallest surviving baby is Rumaisa Rahman, who was born in Illinois in 2004. She was born at 25 weeks and 6 days gestation.
6. 5 years - the age of the world's youngest mother
Lina Medina, from Peru, gave birth to a 2.64kg baby in 1939. She was five-years-old. Her parents had assumed she had a tumour and were shocked to discover that she was seven months pregnant when they took her to hospital.
7. 69 - the age of the world's oldest mother
Rajo Devi Lohan became the world's oldest first-time mother after IVF treatment at age 69 in 2008.
8. 8 - the most children delivered during a single birth to survive
Nadya Suleman gave birth to six boys and two girls in California in 2009. She is often referred to as 'Octomom' and has 14 children in total.
Photo / AP
9. 1 in 11 million - chances of having identical quadruplets
There are around 500 quadruplets born each year, but the chances of them being identical - that is, a single egg fertilised by a single sperm, are one in 11 million. Ornsee Khamsa and Verek Muy gave birth to Preana, Audreana, Natalie and Melody in 2002 in California.
10. 4.3 - the number of babies born each second
Photos / Thinkstock