12-week-old foetus, 3D ultrasound scan. At this stage, during the 12th week of development, the foetus measures around 5 centimetres in length. 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.