Week 31: your baby is the size of an Atlantic horseshoe crab

Your fetus now weighs about 1.5 kg, the weight of an Atlantic horseshoe crab, on the heavy side for a male and mid-range for a female, who is the larger sex. It is not a crab. It is closer to a spider, and its lineage goes back about 445 million years, which puts its relatives on the seafloor long before trees existed.

It has ten eyes scattered across its shell and tail, and blood that runs blue because it carries copper instead of iron. That blood clots instantly around bacterial toxins, so it is harvested to test many vaccines and implants for contamination, then most of the animals are returned to the sea.

On spring high tides they come ashore in the hundreds of thousands to spawn, and shorebirds time their entire migration to arrive when the eggs do.

Meanwhile, inside you

Your fetus this week is going through major brain development and can process information from all five senses. They are also making breathing movements, practicing with amniotic fluid for the real air to come.