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NHS Due Date Calculation:
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The NHS due date calculation estimates a woman's expected delivery date by adding 280 days (40 weeks) to the first day of her last menstrual period (LMP). This is known as Naegele's rule and is the standard method used by the UK National Health Service.
The calculator uses the simple formula:
Where:
Explanation: This calculation assumes a regular 28-day menstrual cycle with ovulation occurring on day 14. The actual conception date is typically about 2 weeks later than the LMP.
Details: An accurate due date helps healthcare providers monitor fetal growth, schedule important prenatal tests, and determine if pregnancy continues beyond term (42 weeks). It's also important for planning maternity leave and preparing for birth.
Tips: Enter the first day of your last menstrual period in the format YYYY-MM-DD. The calculator will add 280 days (40 weeks) to this date to estimate your due date.
Q1: How accurate is this calculation?
A: Only about 5% of women deliver on their exact due date. Most deliver within 2 weeks before or after the estimated date.
Q2: What if my cycles aren't 28 days?
A: The calculation may be less accurate if your cycles are significantly longer or shorter than 28 days. Your midwife may adjust the date based on ultrasound measurements.
Q3: When will I have a dating scan?
A: The NHS typically offers a dating scan between 8-14 weeks which can provide a more accurate due date based on fetal measurements.
Q4: Can the due date change?
A: Yes, healthcare providers may adjust your due date based on ultrasound findings, especially if there's more than a week's difference from the LMP calculation.
Q5: What if I don't know my LMP?
A: If you're unsure of your LMP, an early ultrasound can help establish your due date. The earlier the scan, the more accurate the dating.