I don't have any experience here, but I've got the report from my 20 week ultrasound here and baby has a BPD of 4.6 cm (so just slightly larger than yours) and it's in the 33% range, so your 3% doesn't sound right at all. I think that if it's on your report that it's a typo. If it's your math, check your ID because you might be me.
Even with that small measurement (and her daddy does have a very narrow face, so maybe that's her deal) my baby's gestational age for that measurement was 19w5d. I had the scan at 20+6, so it wasn't off by much.
Here's a website I found really informative and helpful when I got my results.
https://www.baby2see.com/medical/charts.html
I found it with a quick search of the term 'BPD measurements' if you don't want to re-type it all. The part that helped most was this:
Biparietal diameter (BPD):
The transverse width of the head at it's widest. We measure from the the leading edge to leading edge of the bones, because this leading interface is most distinct. Since the head is oval, the error induced by small errors in positioning is small, making for a repeatable, robust measure.
Head size is determined largely by brain growth which is relatively independent of nutritional (maternal/placental insufficiency) growth ******ing processes, and head growth is often relatively "spared" in such growth ******ation. (Okay, I know that's usually an offensive word, but it's also a medical term for being behind!)
The BPD best used after 12 weeks.
Accuracy is +/- 1.1 week 14 - 20 weeks,
+/- 1.6 weeks 20 - 26 weeks,
+/- 2.4 week 26 - 30 weeks,
and +/- 3 - 4 weeks after 30 weeks.
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So at a 20 week scan, it could be 'off' the baby's gestational age by nearly 2 weeks!
If the person doing the scan didn't call someone else in to talk to you, I wouldn't worry. If something wasn't right, they'd have spoken to your midwife (who should, by damn, know at least a little about these measurements!) or your OB-GYN about following up on it.
Anyway, the site also mentions that a baby is considered 'normal' if they fall between the 10th and 90th percentile range. We're all different, hey!
Still, if you're worried, do call and follow up on it yourself. If the person who does know these measurements tells you it's normal, trust him/her and relax so you can enjoy your baby before the midnight screaming starts!
