Placental Migration

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The growth of the lower segment of the uterus in the third trimester is important.

A placenta that appears low-lying earlier in pregnancy may be much more than 2 cm from the os after 34 weeks – so called 'placental migration'.

It is therefore important to examine the placenta again in the third trimester if an earlier scan suggests it is low-lying. A very useful feature is that if the placenta reaches the fundus in the second trimester, it is very unlikely to be low-lying at term.

These images show long axis scans of a fetus at 20 weeks and then again at 32 weeks. Note how the placenta appears as if it might be lying low on the first scan but on the later scan is well away from the os.