26-week-old foetus, 3D ultrasound scan. At this stage, during the 26th week of development, the foetus measures around 36 centimetres in length. The eyes have developed and the lungs are starting to form air sacs. 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.