Does anyone know anything about U/S ratios?! PLEASE help!!

Summer
So, I got the results back from my 20 week ultrasound and at the bottom it says, "Head circumference/abdominal circumference ratio borderline low. Recommend continued attention." 
I am now worried bc I have no clue what that means or why... I haven't spoke with my doctor yet, because the place I go through has it setup online for you to receive your test results, etc. so I can see this stuff before my next doctor visit. 
Should I be worried? Does anyone know anything about this? 😅
Here are the measurements:
BPD 4.2 cm 18w4d (16w6d-20w2d) Hadl BPD 37% FL/BPD 0.70
HC 16.0 cm 18w6d (17w2d-20w2d) Hadl HC 43% FL/AC 0.19
AC 15.2 cm 20w3d (18w2d-22w4d) Hadl AC 79% HC/AC 1.05 (1.06 - 1.25*
FL 2.9 cm 18w6d (17w1d-20w5d) Hadl FL 45% CI 0.70 (0.70 - 0.86)