Small head measurements at 32 weeks pregnant

Hi all.I'm really looking at some reassurance here.I went for a placenta check today as it was low lying at my 20 week scan.It's still low lying which is a little disappointing as I really wanted a homebirth this is my 4th pregnancy .Buy it have totally stopped thinking about that after I was informed they found the baby's head to be small 3rd centile!I had a scan at 23 weeks also because my 3rd child was born with microphthalmia and one of his eyes is small and blind. I have had 3 scans before this one and all seemed fine head wise also and now they seem concerned.It really freaks me out it could be something associated with micro and would just like to know anyone else who was told there child's head was on the small side later in pregnancy.I'm hoping it was just in a low position and they got the measurements wrong.I go back for another scan next Thursday but I'm just in bits and can't think properly now.