Week 9: your baby is the size of a tongue-eating louse

Your fetus is now about the size of a tongue-eating louse, a parasitic isopod (a relative of the woodlouse in your garden) whose adult females reach nearly 3 cm. What it does for a living is the stuff of nightmares and one of the most remarkable feats in all of parasitology.

It swims in through a fish's gills, latches onto the tongue, and drinks its blood until the tongue withers and drops off. Then it simply stays, gripping the stub, and becomes the fish's new working tongue for the rest of its life.

It is one of the only animals known to take over the function of an organ it destroyed, and the fish carries on feeding with the parasite in place, though studies find parasitised fish in poorer condition than their neighbours.

Meanwhile, inside you

Your fetus this week has completed the essential body plan. From here on the work is mostly growth and refinement rather than building brand-new parts.