It’s the first few days of the pregnancy that determine whether you are one person, or two or three, or even four. One thing that surprised me is that human embryos actually hatch, like chickens hatch out of an egg. No one knows how or when multiples are actually made, but one new theory is that if you don’t hatch properly, that probably leads to the creation of twins and triplets.
In the second month you start to see the formation of fingers and toes. There is a wonderful gene called “Sonic hedgehog”, which determines how many fingers and toes you end up with. If your genes are ever so slightly different, and mutations occur, you can end up with more than ten of each. I visited a remarkable family in Brazil who all have six digits on each hand, and they celebrate that fact. No one knows why we normally have fivefingers. It makes you wonder whether our lives would be better if we had six. Would our music be more interesting? Would our mathematical counting systems be totally different?
Are you going to be right-handed or lefthanded? Nobody really knows why some people are left-handed. There are myths, like lefthanded people having shorter life expectancy, which aren’t true. But they do tend to be a bit shorter, and more prone to autism. Ultrasound images of foetuses have shown a preference for the right hand at 11 weeks, long before the brain itself shows any left–right differences. So it could be a fundamental difference in how the left and right arms are built that creates handedness, not the brain.
It’s a big month for the skin – at first it’s totally transparent and over this month it develops fine fur-like hair called “languno”. Sweat glands grow and melanocytes, cells that give skin its colour, colonise the skin from the tissue underneath.
It may be the case that typically male behaviours, such as risk-taking, are programmed into your brain by the testosterone it receives during pregnancy. Have a look at the length of your ring finger, compared with your index finger. Your ring finger grew long if you received more testosterone in the womb. This marker has been linked to people displaying aggressive, risk-taking behaviour, the kind of thing that we expect from people such as City traders.
Your body obviously has no bones at the very start of your existence, only cartilage that slowly begins to harden as the pregnancy progresses. Bone cells within the foetus create hard bone, laying it down like cement. By this point, when you are approaching the end of the second trimester, most of your bones have calcified, but all of them still have soft parts that allow you to keep growing.
Most of the brain’s growth is now in the wiring – fatty sheaths are wrapped around the brain cells, insulating them so they can send signals around your head. You’re making an estimated 100 billion new connections every single day, and you’re beginning to lay the foundations for one of the most important things in the future: your memory.
It’s fattening-up time and the foetus gains weight rapidly, building up a big fat reserve and beginning to look like a portly baby, rather than a wiry foetus.
Your lungs are the last organs to form, which makes perfect sense because they’re not needed in the womb. To complicate matters further, they grow while you’re still encased in liquid, but it’s critical that they work as soon as you are exposed to air. Unborn babies practise breathing, inhaling and exhaling the amniotic fluid that surrounds them. This becomes a critical problem at birth, because they have to quickly get rid of that fluid, so you can draw your first breath.
As told to Patrick Foster
Countdown to Life: the Extraordinary Making of You begins on BBC1 tonight (Monday 14th September) at 9.00pm