What’s one of the first things we want to find out when we discover we’re pregnant? Or the first thing someone asks when they find out you’re expecting?
“When is your baby due?”
We pull out our calendar or open an app, enter the date of our last period, and voilà—our due date appears!
But did you know that only 5% of births happen on their due date? I’m sure you’ll agree that’s not a very accurate calculation. With odds like that, we should really question how accurate that date is.
What Is a Due Date and Why It Might Be Wrong
A due date marks 40 weeks from the first day of your last menstrual period (LMP). But pregnancy isn’t a one-size-fits-all process at all. We just need to look at nature to see examples of that. Research shows that the length of pregnancy can vary by as much as 37 days due to several factors:
Embryos that implant later tend to be carried longer
Older women often have longer pregnancies
First-time pregnancies can last longer
Family history (your mother’s pregnancy patterns)
Environmental stress near the end of pregnancy
And despite all this, being offered a stretch and sweep or induction for being “overdue” may still occur around the 40 week mark. However, guidelines like those from the World Health Organization suggest waiting until 42 weeks+1 day, unless there’s a medical reason for earlier intervention.
The History of Due Dates
In ancient cultures and writings, we read that pregnancy lasted ’10 moons’. This would make pregnancy last approximately 295 days. Today, due dates are typically calculated using Naegele’s Rule. developed by a German obstetrician (Dr Franz Naegele) in 1812. He based his formula on only 100 women and assumed that pregnancy lasts 280 days from the first day of the last menstrual period.
But here’s the catch:
Using the wisdom of our ancient ancestors, pregnancy would last approximately 295 days (not 280 days)
Not all women have a 28-day cycle
Not all women ovulate on day 14
- Allowances like race, age, familial history,and number of births, are not taken into consideration
Looking at all these variables, it’s clear: Dr. Naegele’s rule is full of holes, and should be taken as a guideline, rather than a date set in stone.
Are Ultrasound Scans Accurate in Determining My Due Date?
Yes – to a point. First-trimester dating scans are slightly more accurate than Naegele’s Rule.
Scans in the first- trimester have a margin of error of about 5-7 days
In the second trimester, the margin widens to 8 days
In the third trimester, it stretches to around 14 days
A 2021 study comparing due dates calculated by LMP vs. a 12-week scan found:
First-time mothers:
50% gave birth by 40 weeks + 5 days
75% by 41 weeks + 2 days
Mothers who’d given birth before:
50% by 40 weeks + 3 days
75% by 41 weeks
Clearly, these findings show that we should treat a due date as a general estimate, not a set-in-stone date that needs medical intervention if this date comes and goes.
How your baby helps initiate labour
Some fascinating science: In the last few weeks and days of your pregnancy, your baby’s lungs are maturing, brown fat is being laid down so he/she can regulate their own temperature efficiently, and the brain continues to rapidly mature. It’s believed that the baby’s lungs release a protein once they are fully developed, which helps trigger labour. So, in many ways, your baby initiates their own birth.
When an early birth is necessary
Every pregnancy is unique, and in some cases, birthing your baby earlier may be the best option for your and/or your baby’s health These include:
Gestational diabetes
Preeclampsia
Placental complications
If these apply to you, your care team may suggest increased monitoring or birth via induction or caesarean section. Take your time to ask questions and, if necessary, do your own research before making a decision.
Going Past Your Due Date
As you can see, due dates are not an exact science. There are accuracy issues that need to be considered when making decisions. Here’s how to stay grounded and empowered if your estimated due date arrives with no signs of labour:
Hold your due date loosely; consider telling people your due month, not the day
Plan to do something fun or relaxing on your due date
- Send a message to family and friends letting them know you will let them know when things have happened, then turn off your phone!
Trust your body and baby – they’ve brought you this far, and know what to do
Learn about your options if offered a stretch and sweep or induction. Should I have an Induction? is a blog to inform you about your choices.
Be patient – it’s excellent practice for motherhood!
Going past your due date can be emotionally tough, especially when family and friends check in constantly and there is huge pressure from the medical team to induce. Surround yourself with support, practice self-care, and try not to stress over the calendar.
I highly suggest reading ‘In Your Own Time’ by Dr Sara Wickham for an evidence and research based approach to due dates.
Final Thoughts on Due Dates and What They Really Mean
Pregnancy is not a mathematical formula. You and your baby don’t need to conform to formulae, graphs or induction dates. Each pregnancy is a unique journey. Your baby is not a train running on a tight timetable – they’re a person arriving in their own perfect time.
If you have any questions or concerns, feel free to drop me an email or book a free 30-minute consultation. I’m here to help you feel informed, empowered, and confident in your pregnancy journey.
Wishing you patience, trust, and peace as you await your little one’s arrival.
