Week 37: your baby is the size of a common cuttlefish

Your fetus now weighs about 2.9 kg, a large common cuttlefish. This week your baby reaches early term, essentially ready and just topping off.

A cuttlefish can change the colour, pattern and texture of its skin in about a second, using millions of pigment sacs it controls like muscles. It does this while being colourblind, reading the world instead through polarised light.

Its pupils are shaped like a W. Inside is the cuttlebone, a rigid gas-filled float that lets it hang motionless in the water. Cuttlefish court in code: a male can flash courtship stripes down the flank facing a female while showing a rival on his other side a female pattern, sweet-talking a mate and fooling a competitor at once. That split signal was described in the mourning cuttlefish; whether this species pulls the same trick is still an open question.

Meanwhile, inside you

Your fetus this week has a firm grip and is practicing all the skills they will need immediately: sucking, swallowing, and breathing. Their brain and lungs are the last organs still finishing up.