Pregnancy
Due date
Naegele's rule adds 280 days to your last period and assumes a textbook 28-day cycle. If yours runs longer, your true date is later — a 35-day cycle moves it by a full week.
How this is calculated
EDD = LMP + 280 days, shifted by cycle length − 28 when the adjusted method is selected. Conception dating adds 266 days; IVF transfers are exact, since the fertilisation date is known rather than inferred.
Common questions
How accurate is a due date?
Only about 4% of babies arrive on the estimated date. It marks the midpoint of a roughly five-week window in which a normal birth is expected, not an appointment. Naegele's rule also tends to date delivery two to four days early.
My cycle isn't 28 days. Does that change the date?
Considerably. Naegele's rule assumes ovulation on day 14 of a 28-day cycle, so a longer cycle means you ovulated later and your true date is later too — a 35-day cycle moves it by a full week. Use the cycle-adjusted method above.
My scan gave a different date than my last period. Which is right?
The scan. A first-trimester dating ultrasound is the most accurate method available and is what your clinician will use if the two disagree, particularly when cycles are irregular or the last period date is uncertain.
What's the difference between gestational and fetal age?
Gestational age counts from the first day of your last period and is what every clinician, scan and milestone chart means by 'how many weeks'. Fetal age counts from fertilisation and runs about two weeks behind. A 12-week scan means 12 weeks gestational, 10 weeks fetal.