U/S for size???

[Shaz]

I'm hoping someone can help me. On another board I frequent, a woman is being told she needs a u/s at 36weeks to determine size of the baby(she is GD). I know that u/s at this late date is notoriously inaccurate, but she wants to know why?? Can anyone tell me WHY it is so inaccurate?? I can give her a ton of examples from my previous client's experience where it was very wrong on size but not the WHY it is so wrong most of the time.

Thank you so much for any help!!

[tea olive]

the basics i know is that the baby size is more consistent earlier in the game like before 20 weeks because it is following the sme growth patterns as other babies. which is based on numbers of cells i guess. the later in the pg the size is more related to nutrition and health factors thus the measurement to determine due date becomes more inaccurate. around 34-36 weeks the baby may gain a pound a week!