Head Measurements

I went in today for an ultrasound (33 weeks, 4 days) because of some medication I’m on and suddenly the baby’s head is measuring super small. Her body is in the 40th percentile and her head was below 10, I think the 7th percentile. When the first tech did the reading it was less than 1 percent. Now they want to do a biophysical every week. I’m totally freaked out. I’m a super small person so part of me wonders if it’s just a miscalculation and they couldn’t get a great read because she is low

In my pelvis. I’m just unsure how everything was fine until now and suddenly it’s cause for concern. Anyone have some reassuring words of wisdom?