I wouldn't be too concerned - they way the baby is lying can also have an impact on the measurement. For example at 29 weeks I measured 31 weeks and at 31 weeks I measured 31 weeks - but my OB wasn't concerned as the baby had moved position.
My midwife said its fine I wouldn't worry.
At 31 wks I measured 32 weeks, and this week (34) I'm back to measuring right. Just the babies position or maybe it had a little growth spurt in the last couple of weeks and will slow down again for a couple of weeks.
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