Uterine fibroid in pregnancy. Coloured 3-D ultrasound scan of a fibroid (upper centre) in the uterus (womb) of a pregnant woman. At bottom is the placenta, which surrounds the foetus and provides it with nourishment and eliminates waste products. A fibroid is a benign tumour of fibrous and smooth muscle tissue. The presence of a fibroid causes problems for a pregnancy and often leads to miscarriage. As the pregnancy progresses and levels of oestrogen increase, the fibroid will grow larger. Ultrasound is a diagnostic technique that sends high frequency sound waves into the body via a transducer. The returning echoes are recorded and used to build an image of an internal structure.