Are Due Dates Accurate? Why Your Baby Probably Won’t Arrive on Time

If you’re pregnant, chances are you’ve got a date circled on your calendar—the magical due date. But are due dates accurate? Only around 5% of babies actually arrive on that specific day, so if yours doesn’t, there’s no need to panic.

In this post, we’ll unpack where the due date calculation comes from, why most babies come earlier or later, and how to prepare for the uncertainty.

So, Are Due Dates Accurate or Just a Guess?

Due dates are based on a calculation from the 1800s, when German obstetrician Franz Karl Naegele created a formula: take the first day of your last period, add one year, subtract three months, and add seven days. This assumes a 28-day menstrual cycle with ovulation on day 14. But here’s the problem: most of us don’t have textbook cycles.

Factors That Affect Your Due Date

• Cycle Length Variations – If your cycle is longer or shorter than 28 days, your due date could be inaccurate.

• Ovulation Timing – Conception doesn’t always happen on day 14.

• First vs. Subsequent Pregnancies – First babies often arrive later.

• Family History – If your mother or sister had longer pregnancies, you might too.

One study (Smith, 2001) found that the median pregnancy length is actually 40 weeks and 5 days, meaning most people are already "overdue" by current medical standards.

What Happens When You Go Over Your Due Date?

For most pregnancies, going past 40 weeks is completely normal. "Post-term" (over 42 weeks) pregnancies are less common but still within the realm of normal.

Why Do Some People Feel Pressured to Induce?

• Some hospitals routinely offer induction at 41 weeks, but this is often based on policy rather than individual need.

• Inductions for "post-dates" make up a significant percentage of medical inductions, yet research suggests many of these labours would start naturally within a few days.

• Your baby might not actually be "late"—just growing at their own pace.

How to Mentally Prepare for an Unpredictable Birth Date

✔ Think of a “Due Window” Instead of a Date – Expect birth between 38 and 42 weeks.

✔ Set Boundaries with Well-Meaning Friends & Family – Tell them your due “month” instead of a specific day.

✔ Know Your Rights Around Induction – Induction is an option, not a requirement. If you’re being offered one, ask:

• What are the benefits?

• What are the risks?

• Are there alternatives?

• What happens if we wait?

• Ask for accurate figures. Don't accept 'the risk doubles', find out the actual percentages!

  
  

Want More No-Nonsense Birth Insights?

Due dates are made-up, but solid birth prep isn’t. If you want straight-talking, evidence-based midwifery tips, follow me on Instagram @mydarlingmidwife for myth-busting, practical advice, and the occasional rant about outdated birth policies.

📍 Got a question? Drop me a message—I love helping people feel confident about birth.