Sometimes, an ultrasound scan is indicated in the third trimester of pregnancy. These scans are useful for those women who have had previous complications of pregnancy such as pre-eclampsia, stillbirth etc. They may require a growth scan because of a medical condition in the mother (eg diabetes or hypertension), or if there is a possible clinical problem related to the pregnancy (eg small for dates). Quite commonly, the placenta is low-lying at the mid-trimester scan and needs to be checked in the third trimester to ensure that it is clear. At this scan, the fetus’ biometric parameters are measured to plot on growth charts, (i.e. the size of the fetal head, abdomen and thigh bone and calculation of an estimate of fetal weight). Amniotic fluid, umbilical artery blood flow and the placenta are also assessed. Colour Doppler pictures of the umbilical cord and fetal brain are obtained. From the pattern of the blood flow in these vessels it is possible to assess if the placenta is working well and that the fetus receives a normal amount of oxygen. Generally, the Doppler equipment and scanning methods used on the patient are the same equipment as for a general ultrasound test. Additional time may be required, however, to perform the Doppler part of the examination. 3D/4D scans will also be performed.
34 weeks’ gestation