Week 19: your baby is the size of an American pika

Your fetus is now the size of an American pika, a round, tail-less ball of fur about 15 to 20 cm long that lives in the boulder fields of high western mountains. It looks like a hamster and squeaks like a squeaky toy, but it is actually a tiny relative of rabbits.

Pikas do not hibernate. Instead they spend all summer frantically harvesting wildflowers and grasses, then stack them into little sun-dried haypiles among the rocks to eat through the winter. A single pika may make hundreds of gathering trips a day, and they will defend a good haypile fiercely.

They are also one of the clearest living thermometers of a warming climate. Pikas can die within hours at temperatures in the high 20s °C, so as mountains warm they are being pushed higher and higher upslope, which makes them a closely watched indicator species.

Meanwhile, inside you

Your fetus this week is developing a greasy protective coating called vernix that shields their skin in the amniotic fluid, and their sensory neurons for touch, taste, hearing, and vision are wiring up in the brain.