Week 26: your baby is the size of a chambered nautilus

Your fetus now weighs about 760 grams, roughly a chambered nautilus. It is a deep-water mollusc and one of the last nautiloids, a lineage that has been drifting through the oceans for around 500 million years, hunting by smell along reef walls at night.

The shell is a spiral of sealed rooms. The animal lives only in the outermost one; the rest hold gas and fluid it can adjust to sink or rise, the same principle as a submarine's ballast tanks. It builds a new chamber as it grows and seals the old one behind it.

Its eye is a pinhole with no lens, open to the sea. It has around ninety tentacles, none with suckers, and it can live fifteen to twenty years, unusual for a cephalopod.

Meanwhile, inside you

Your fetus this week is opening their eyes for the first time, having had them fused shut for months. They are also developing regular sleep and wake cycles, including REM sleep, which means they may be dreaming.