20w scan hc measuring in 5th centile - worried

[Geobaby]

Hello. I had my 20w scan yesterday and whilst we were assured that all the baby's organs & brain looked normal, the hc was in the 5th centile (160.8 mm). They said at this stage we should not worry, but we are to go back in 4 weeks for another scan to check growth. What concerns me is that the ac was 150.8 mm (which looking at the tiny graph appears to be in the 60th (ish) centile. In my not at all expert opinion there seems to be a large difference BTW hc and ac. Does anybody else have experience of this? I know they said not to worry, but I am. A lot. My first baby measured big and in proportion (I think) and was 9lbs at birth. Both my husband and I are tall (5'10" and 6'), he's built like a rugby player and I'm not slight, so I wouldn't have thought that we'd have a small baby! Can anyone put my mind at ease? Thanks.

Did you have a dating scan?

Yes, my dating scan at 12w matched exactly with my dates based on my LMP.

My first DD's HC was something like the 0.5th centile from 6 weeks old though she was 50th for length. I worried myself stupid about it (I too am tall). Turns out she just has a small head (still does! Can wear babies' hats at age 3!)

I appreciate this wasn't at a 20 week scan. My good friend's baby had a HC that was below the zero centile at a scan - was reassured and all was and is well. Interestingly her head is average size now.

I think the fifth centile is just on the small side and its good they are double checking to give you further reassurance.

HTH!

It could have been a measurement error. Did the sonographer repeat it?

Thanks oohdaddypig. That gives me some hope.

Roshbegosh - I don't know how many times she took the measurements. She seemed experienced and so I'm sure she would have taken more than one set once she realised it was a bit small. She used the BPD measurement to calculate hc. Not sure if this can make the results a bit less accurate.

I'm going to try and not worry about it for the next 4 weeks. I can feel lots of kicks and so we I'm going to try and focus on the positive.

HC can't be calculated from BPD. Bloody hell.

Sorry but I can't believe it.

Not an expert on fetal measurement, but have been for regular scans with fetal medicine consultant, and we were chatting about centile measurement. ( I am a bit of a statto. ) She made the point that each measurement is pretty unreliable. I had been telling her about my resolutely 50th centile first baby, and how frustrated my then doctor was that someone who would understand that it's ok to have a baby on the 5th or the95th ended up with a 50th.

So in short, 5th is completely normal and the consultant clearly thought that the particular measurements were potentially spurious.