1. What is the NHS Due Date Calculator?

The NHS Due Date Calculator estimates when your baby is likely to be born based on the first day of your last menstrual period (LMP). This follows the standard NHS method of calculating due dates, which adds 280 days (40 weeks) to the first day of your last period.

2. How Does the Calculator Work?

The calculator uses the standard NHS formula:

Where:

• \( LMP \) — First day of last menstrual period
• \( 280\ days \) — Equivalent to 40 weeks gestation

Explanation: Pregnancy is calculated from the first day of your last period because it's often difficult to know exactly when conception occurred, while most women remember when their last period started.

3. Importance of Due Date Calculation

Details: Knowing your estimated due date helps healthcare providers monitor your pregnancy progress, schedule important tests at the right times, and identify if your pregnancy is progressing too slowly or quickly.

4. Using the Calculator

Tips: Enter the first day of your last menstrual period in the format YYYY-MM-DD. The calculator will show your estimated due date and current gestational age in weeks and days.

5. Frequently Asked Questions (FAQ)

Q1: How accurate is the due date calculation?
A: Only about 5% of women deliver on their exact due date. Most deliver within 2 weeks before or after.

Q2: What if I don't know my LMP or have irregular periods?
A: An early ultrasound scan can provide a more accurate estimate in these cases.

Q3: Why does the NHS use 280 days?
A: This is based on the average 28-day menstrual cycle with ovulation occurring around day 14.

Q4: Does this work for IVF pregnancies?
A: For IVF pregnancies, the calculation is based on the embryo transfer date plus 266 days (or 38 weeks).

Q5: When will my midwife change my due date?
A: If your first ultrasound scan estimate differs significantly from your LMP-based date (by more than 7 days), your midwife may adjust your due date.