Pregnancy Due Date Calculator (EDD): Estimate When Your Baby Might Arrive

Finding out you’re pregnant often comes with one big, immediate question: “When is my baby due?” A due date (also called your Estimated Due Date / EDD) gives you a helpful timeline for planning antenatal appointments, tracking pregnancy milestones, and getting ready for birth.

But here’s the key thing to know up front: a due date is an estimate, not a deadline—pregnancy length naturally varies, and only a small percentage of babies arrive on the exact date predicted. (JAMA Network)

This guide explains how due date calculators work, how accurate they are, and what to expect next—especially if you’re in the UK.

How a pregnancy due date calculator works

Most due date calculators start with the first day of your last menstrual period (LMP) and then estimate your EDD by adding 280 days (40 weeks)—with an adjustment if your cycle is shorter or longer than the “average” 28 days. (Johns Hopkins Medicine)

That’s also why many calculators ask for:

• Day 1 of your last period
• Average cycle length (in days)

The NHS calculator, for example, highlights that cycles aren’t always 28 days and notes that pregnancy is often considered to span roughly 37 to 42 weeks from the first day of your last period. (nhs.uk)

What competitors do well (and what you should match or improve):
The strongest due date calculator pages explain multiple calculation options (LMP, conception date, IVF transfer date, ultrasound date), emphasize that the due date is an estimate, and often offer a pregnancy timeline with milestones.

The most common ways to calculate your due date

1) Last menstrual period (LMP) method (most common)

If you know the first day of your last period, most providers and calculators estimate your due date using:

• EDD = LMP + 280 days (40 weeks) (Johns Hopkins Medicine)

You might also see Naegele’s Rule, a classic shortcut:

• Add 7 days
• Subtract 3 months
• Add 1 year (Johns Hopkins Medicine)

Best for: fairly regular cycles and when you don’t know the exact conception date.

2) Conception date method (if you know it)

If you tracked ovulation closely (or have a known conception date), many calculators use:

• EDD = conception date + 266 days (38 weeks) (perinatology.com)

Best for: irregular cycles when you have reliable ovulation/conception tracking.

3) IVF transfer date method (high precision)

If you conceived via IVF, you often know timing more precisely. Common estimates include:

• Day-3 embryo transfer → add 263 days
• Day-5 embryo transfer → add 261 days (Texas Health Resources)

Best for: IVF pregnancies (your fertility clinic can confirm the dating used in your care plan).

4) Ultrasound dating scan (often the most accurate early on)

Clinical guidance commonly recognizes first-trimester ultrasound as the most accurate way to establish or confirm gestational age (up to 13 weeks + 6 days), with accuracy commonly described in the ±5–7 day range depending on timing and measurement. (healthcare.uiowa.edu)

Best for: irregular periods, unknown LMP, or when LMP and growth measurements don’t match.

How accurate is a due date, really?

Due dates are useful—but biology is not a calendar invite.

Research and large datasets consistently show:

• Only around ~4% of people deliver on their estimated due date. (JAMA Network)
• Pregnancy length can vary naturally by weeks, even with good early dating. (JAMA Network)

A more realistic way to think about your EDD:

• It’s the center point of a likely arrival window (not a guarantee).

Why your due date might change later

Yes—your EDD can change, and it’s not automatically a cause for alarm.

Common reasons include:

• Irregular cycles (ovulation didn’t happen around day 14)
• Uncertain LMP
• Dating scan measurements suggesting a different gestational age (nhs.uk)

If your clinician adjusts your due date after a scan, it’s usually because they’re aiming for the most reliable estimate for your care schedule.

UK pregnancy timeline: what happens after you calculate your due date?

If you’re in England (and similarly across the UK with local variations), your due date helps plan key appointments. The NHS outlines typical milestones like:

• 8–12 weeks: booking appointment
• 11–14 weeks: dating scan (often called the “12-week scan”)
• 18–21 weeks: anomaly scan (“20-week scan”) (nhs.uk)

The NHS also explains that the dating scan is typically offered around 10–14 weeks and is used to estimate a more reliable due date based on measurements. (nhs.uk)

How to use your due date calculator

If you’re building or improving your calculator page, keep the steps ultra-clear (and friendly):

1. Enter the first day of your last period
2. Select your average cycle length (if you’re unsure, 28 days is a common default)
3. Click Calculate to see your:
   • Estimated due date (EDD)
   • Current pregnancy week
   • Trimester
   • Key milestone dates (optional but high-value)

Also include a visible note: “This is an estimate, not a diagnosis.” (This language is common and trust-building.) (nhs.uk)

Trust & safety note

This content is for general education and planning support. Your midwife, GP, or obstetric team will confirm your pregnancy dating and due date—often using your dating scan and clinical history. (nhs.uk)