12 weeks pregnant: what is happening this week

The figure crosses into centimetres this week: 5.4 cm, plum-sized, measured head to bottom. Internal organs and muscles have grown enough that a heartbeat shows on ultrasound. The skeleton is converting from soft tissue into hardening bone. Sex organs have formed, though they are generally not visible on a scan at this stage. For most people this is scan week, and it is worth being precise about what a dating scan does. It measures the foetus and computes a gestational age from that measurement, and where that age disagrees with the date arithmetic by more than a few days, the scan wins. ACOG is unambiguous about this and reports that in one study around 40 per cent of women had their due date revised after a first-trimester scan. A due date that moves is not an error being corrected in you; it is a better measurement replacing a worse one. Nausea often begins to lift around now and appetite returns, which after six weeks of negotiating with food can feel disproportionately good. Folic acid, on NHS guidance, runs to the end of this week and then stops.