Your Pregnancy Due Date: Common Mistakes, Tips and FAQs

Introduction

Health numbers are useful when you understand what they measure and what they don't. Many popular formulas are estimates with real limits.

This article covers the most common mistakes people make with your Pregnancy Due Date, what really moves the result, and answers to the questions we hear most often.

Why it matters

Understanding your figures can help you set sensible fitness goals, track change over time and know when to ask a health professional for advice.

How it's calculated

The due date is 280 days (40 weeks) after the first day of your last period. Your dating scan may adjust this.

What changes the result most

The result depends on the choices you make in the calculator, so it's worth trying several combinations. In a typical example, estimated due date is Sun, 6 December 2026.

Common mistakes to avoid

1. Treating a single number as a complete picture of health.
2. Using formulas designed for adults to assess children.
3. Measuring inconsistently, such as at different times of day.
4. Changing diet or exercise drastically based on one estimate.

A quick example

With the inputs below, estimated due date comes out at Sun, 6 December 2026.

Tips

• The NHS uses BMI as a screening tool, but waist measurement also matters.
• Estimates like BMR and body fat can vary by 5–10% from lab measurements.
• Track trends over weeks rather than day-to-day changes.
• Speak to your GP or midwife about any results that worry you.

Frequently asked questions

What's the quickest way to work out your Pregnancy Due Date?

Use our free pregnancy Due Date Calculator. It applies this method automatically: The due date is 280 days (40 weeks) after the first day of your last period. Your dating scan may adjust this.

Is this medical advice?

No. These tools give general estimates. For medical advice, speak to your GP, pharmacist or NHS 111.

Can I use these for children?

No. Children's BMI and health measures use age- and sex-specific charts. Use the NHS healthy weight calculator for children.

Why use stone and pounds?

Many people in the UK still think in stone and pounds, so we accept those units and show metric too.

Related calculators

• BMI Calculator (Stone and Pounds)
• BMR Calculator
• Calorie Calculator
• Body Fat Calculator
• Healthy Weight Range Calculator
• Water Intake Calculator

For the full method, read How to Calculate Your Pregnancy Due Date in the UK: Step-by-Step Guide.