A hiccup is a reflex contraction of a muscle – just like when you hit your knee and it kicks. Distension of your stomach next to your diaphragm (a large muscle like a frisbee in the bottom of your chest that helps you to cough, breathe etc.) contracts, causing a hiccup!
Ultrasound scans show that two-month old babies hiccup in the womb, before any breathing movements can be seen.
One theory is that hiccups prepare the baby’s muscles for breathing after it’s born. Another is that they stop the fluid that surrounds it in the womb – amniotic fluid – from entering the lungs.
But neither of these theories really works. If hiccups were to keep fluid out of the lungs, for instance, they should be a sort of cough out, not a breath in.
So a few years ago a team of French scientists suggested that hiccups might be linked to evolution – and the fact that our ancestors lived in the sea.
Hiccups might be a leftover from when our ancestors had gills to help them breathe. There are lots of similarities, they say, between hiccuping and the way tadpoles, for instance, push water across their gills.
But why would we still have hiccups 370 million years after our ancestors left the sea and lost their gills?
Maybe it’s because hiccups have been adapted, the scientists suggest, to prepare the baby for something else mammals do, even on land – sucking milk.
Other scientists agree that all this is a possible explanation of hiccups. “But it’s going to be very tough to prove,” they say.