28 week old fetus. Colored ultrasound scan of a 28 week old healthy fetus in the womb, sucking its thumb. The head of the fetus is at center, with its arm at lower right. The crown-rump length (sitting height) of a fetus aged 28 weeks is about 37 centimeters, and its weight is about 900- 1000 grams. Ultrasound scanning, using high- frequency sound waves, is a diagnostic tool to confirm fetal age and detect any malformations.