Week 5: your baby is the size of a chigoe flea

You likely have not even missed your period yet, and already your embryo is the size of an adult chigoe flea, also called a jigger. At about a millimetre long it is the smallest flea in the world, and one of the more unwelcome.

The female burrows head-first into the skin of a mammal, often into a human foot, and simply stays. Over 8 to 10 days she swells with eggs to the size of a pea, up to 80 times her original size, before releasing them back into the world.

It is a vivid, if slightly alarming, example of how much a tiny animal can balloon in size, which is more or less the theme of your next 40 weeks. Your embryo is about to do the same, minus the burrowing.

Meanwhile, inside you

Your embryo this week is mostly about a beating heart. The tube that will become the heart starts pulsing around now, weeks before it looks like anything a doctor could point to.