Wouldn’t it be lovely to know exactly what date your baby will appear. It’s a lovely benefit of having an induction or planned C section – that you can actually plan more accurately!
You may also have been somewhat confused to be given one due date, and then told a different day a few weeks later! So here is more about how we decide which to follow!
The due date is important:
✔️To allow you to plan your life at that time
✔️To establish when to have certain screening tests during pregnancy
✔️To be able to assess the baby’s growth along pregnancy
✔️To decide at what point to advise delivery of baby if spontaneous labour hasn’t happened.
In early pregnancy, your due date can be predicted using the first day of your last period (LMP) by adding 280 days. However, once you have a 12 week (dating) scan then the date is determined from that scan onwards.
Why don’t we use the LMP?
🤷🏼♀️Around half of women cannot recall this accurately
🤷🏼♀️If your cycle is irregular, the LMP method relies on the fact that you will have ovulated at day 14 which may not be the case
🤷🏼♀️Remember this date would be 2 weeks BEFORE the date you think you conceived. So even if you know the exact date of intercourse that month (or the date of embryo transfer if IVF) this still needs to be adjusted.
🤷🏼♀️It’s impossible to determine the exact day of fertilisation with natural conception because of the variation in the day the egg is released and also the fact that sperm can hang around for 3 days or so.
How do you decide due date from scan?
At your dating scan, usually done between 10 and 13+6 weeks of pregnancy, a measurement will be taken of the largest diameter of the fetus, known as the CRL.
How accurate is the date from ultrasound?
If the CRL is used then this can predict the date of delivery to within 5 days before or after. In one UK study quoted in the NICE guideline, delivery will occur within this window in 52% of cases, compared with 37% using LMP alone. In addition, using the dates from ultrasound significantly reduces the rates of induction of labour for prolonged pregnancy. Therefore the NICE UK guidelines advise ultrasound over LMP to determine your due date.
References:
NICE guideline evidence: Antenatal care routine care for the healthy pregnant woman – https://www.nice.org.uk/guidance/cg62/evidence/full-guideline-pdf-196748323
2 Comments
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Comments are closed.My wife is pregnant, so we want to make sure that we know when the baby will come. It makes sense that due dates can change as time goes on! Maybe it would be a good idea to talk to an OBGYN to see if the day has changed for us.
I like that you mentioned that it’s common to be confused about a given due date only to be told a different day later. I’ve been a little worried about the whole pregnancy process as it’s my first time. Maybe I should go with my wife to her next OB Doctor’s appointment and see if that helps me understand how to prepare for it.