Week 3: your baby is the size of a giant rotifer

Your embryo is now the size of a giant rotifer, which sounds like a joke until you meet the competition. Limnias ceratophylli tops out around half a millimetre, which puts it at the big end of a group where most species are smaller still. Your embryo is roughly there too.

Rotifers are microscopic animals that beat a crown of cilia to sweep food into their mouths, which is where the name comes from: rotifer means wheel-bearer. The cilia fire in sequence rather than actually spinning, but under a microscope the illusion is perfect, two glassy wheels turning on the front of the head.

Limnias spins itself a slender tube, settles inside, and never moves again. Some do it on a crocodile. Limnias ceratophylli has been reported living on the skin of Amazonian caimans and crocodilians, riding the animal and filtering its meals from the water as its host swims, which is a considerable gamble for a passenger half a millimetre long.

Meanwhile, inside you

So this week you and a crocodile have something in common: a very small passenger that is mostly just getting comfortable. Your embryo is a hollow ball of cells called a blastocyst, newly burrowed into your uterine wall.