Estimates of the number of sextuplets born worldwide vary, but some say there are fewer than 200 sets.
The birth of sextuplets is very rare, with some experts putting the odds at one out of 4.5 million pregnancies.
Some babies born as part of sextuplets have died shortly after birth but advances in neonatal care means they are more likely than ever before to survive.
The first known sextuplets in Britain were born in 1968 in Birmingham.
Sheila Thorns delivered four boys and two girls by Caesarean. Three of the babies survived.
The last sextuplets born in the UK were the Walton sisters, who are the world's first surviving all-female sextuplets.
They were delivered by Caesarean section in 1983 to Graham and Janet Walton, who had spent years trying for children and had undergone fertility treatment.
Hannah, Lucy, Sarah, Kate, Jenny and Ruth arrived to a media storm although their parents have always stressed they want the girls to be treated as individuals.
They celebrated their 25th birthdays last November with a quiet family party at home in Wallasey, Merseyside.
In 1996, British woman Mandy Allwood famously gave birth to eight children following fertility treatment. All of them died soon after delivery.
The most recent high-profile case of multiple births involved Nadya Suleman, who delivered octuplets in January in California.
She was the second person recorded in the US to have delivered a set of living octuplets and her six boys and two girls were born nine weeks premature following IVF.
She has publicly defended herself after it emerged she has given birth to a total of 14 children.
Around 130 sets of triplets and around 11,000 sets of twins are born each year in the UK.
The number of multiple births is on the rise thanks to fertility treatment and also mothers delaying motherhood, which makes them more likely to conceive multiples.
Multiple pregnancies are linked to health problems for both mother and babies, with around half of twins born premature requiring specialist hospital care.
Babies born from multiple births are more likely than singletons to suffer low birthweight, impaired vision, breathing problems and conditions like cerebral palsy.
They are also more likely to suffer long-term learning difficulties than other children.
Mothers are more likely to suffer pre-eclampsia in pregnancy - a condition linked to high blood pressure - and heavy bleeding.
Multiple pregnancies also carry a higher risk of miscarriage.
In November 1997, the world's first recorded case of surviving septuplets occurred when three boys and four girls were born to a couple in the US.