Accuracy of BPD for Dating the Pregnancy
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Understanding the limitations of measuring the baby’s head in order to date the pregnancy is very important. The accuracy is best from 12 weeks to 20 weeks although it can still be used to date the pregnancy up to 24 weeks.
Look at the chart opposite.
When the BPD measures 40 mm, you can see that it predicts the age of the baby as between about 16 weeks and two days and 18 weeks and 4 days. You would normally date the pregnancy at the average point – 17½ weeks.
However, when the BPD measures 90 mm, you can see that although the average prediction would be 36½ weeks, the range of normal would be between 33 and 40 weeks!
It is important to remember this. Ultrasound is very accurate for dating the pregnancy before 24 weeks but in late pregnancy accuracy is much less.
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