Ultrasound scans of a 12-week-old foetus. 2D (grey) and 3D (orange) ultrasound scan footage of a foetus that has reached the 12th week of development. At this stage the foetus measures around 5 centimetres in length. At around this time, the brain significantly increases in mass and becomes structurally developed. Birth occurs at around 38-40 weeks. Ultrasound scanning uses the reflected echoes of high-frequency sound waves to image an unborn baby. This safe, non-invasive method is used routinely in pregnancy to assess the growth and health of the developing foetus, and detect any abnormalities.