Are 30+ week ultrasound measurements accurate to date a pregnancy?

Yes and no: They are accurate, but only to around +/- 3 weeks.

Not exactly: while an ultrasound in the third trimester can give you an idea based on size of the fetus, they are only accurate +/- 3 weeks. If the fetus is having growth restriction or is large for age, they will be off. If there is a fluid problem, they will be off. The best time in a pregnancy to obtain dating via ultrasound is the first trimester. Good luck!