Due date | Mumsnet

Are your children’s vaccines up to date?

Pregnantpolly · 05/06/2022 12:09

Scans help with dating as they base the CRL measurement and align it with gestational age. From that they can calculate when you'll be 40 weeks.

Original poster

Bailyray86 · 05/06/2022 12:11

@Pregnantpolly My last period was April 11th but since then I’ve had all negative tests up till the 19th may they have all been completely negative so I’m confused really?

OP posts:

Weathergirl1 · 05/06/2022 12:13

They measure the crown rump length of the foetus and then extrapolate to a due date from that. It assumes that your baby is bang on the 50th centile (average) which is why EDD from scans can be different if you know when you ovulated or had IVF transfer.

LMP based EDD often won't match because those assume ovulation is at day 14 of the cycle which won't be the case for all women.