EDD calculation for nursing students: Naegele’s rule, POG and worked examples
Calculating the EDD (expected date of delivery) and the POG (period of gestation) by hand is one of the first skills taught in obstetric nursing and midwifery — and a favourite exam question. This guide walks through both, with worked examples and the mistakes that cost marks. When you’re ready, test yourself with the Naegele’s rule practice questions.
The terms you need
- LMP — last menstrual period: always the first day of the last period (see LMP meaning).
- EDD / EDC — expected date of delivery (or confinement): the due date, 280 days after the LMP.
- POG / GA — period of gestation or gestational age: how far along the pregnancy is, in completed weeks plus days, counted from the LMP.
Naegele’s rule, step by step
EDD = LMP + 7 days − 3 months + 1 year. Work it in that order and write each step down — examiners often give marks for the working.
- Write the LMP. Example: 14 August 2026.
- Add 7 days to the day. 14 + 7 = 21 → 21 August 2026.
- Subtract 3 months. August − 3 = May → 21 May 2026.
- Add 1 year. 2026 + 1 → EDD 21 May 2027.
The same answer comes from adding 9 months and 7 days. Counting exactly 280 days can land a day or two away because months have different lengths; both are acceptable, but follow the method your syllabus uses.
Worked examples with a month roll-over
Example 1 — LMP 27 May 2026. 27 + 7 = 34, and May has 31 days, so it becomes 3 June 2026. June − 3 months = March → 3 March 2026. + 1 year = 3 March 2027.
Example 2 — LMP 2 February 2026. 2 + 7 = 9 February. February − 3 months = November of the previous year → 9 November 2025. + 1 year = 9 November 2026. (With January–March LMPs, adding 9 months directly is quicker: 9 February + 9 months = 9 November 2026.)
Example 3 — LMP 25 December 2026. 25 + 7 = 32 → 1 January 2027. January − 3 months = October 2026 → 1 October 2026. + 1 year = 1 October 2027.
Calculating the POG
To find the period of gestation on a given date, count the days from the LMP to that date, divide by 7, and write the answer as completed weeks plus the remaining days.
Example — LMP 14 August 2026, visit on 14 November 2026. August 14–31 is 17 days, September 30, October 31, November 1–14 is 14: 17 + 30 + 31 + 14 = 92 days. 92 ÷ 7 = 13 remainder 1, so the POG is 13 weeks 1 day, written 13+1. The gestational age calculator checks this instantly.
Adjusting for cycle length
Naegele’s rule assumes a 28-day cycle with ovulation around day 14. For a longer cycle, add the extra days to the EDD; for a shorter one, subtract them. A 32-day cycle (+4) moves the 21 May 2027 EDD above to 25 May 2027. Ask whether the cycle is regular — with irregular cycles, LMP dating is unreliable and an early scan matters more.
When ultrasound overrides the LMP
ACOG recommends confirming LMP dating with ultrasound, ideally in the first trimester when the crown-rump length is most accurate. The due date is changed to the scan date if the two differ by more than 5 days before 9 weeks, or by more than 7 days between 9 weeks and 13 weeks 6 days. Once set by an early scan, the EDD isn’t usually changed again. The CRL calculator converts a CRL in mm to gestational age.
Trimesters and term categories
Commonly: first trimester 0–13 weeks, second 14–27 weeks, third 28 weeks to birth (some syllabi use 12 and 26 weeks as the cut-offs). ACOG’s term categories are: early term 37 0/7–38 6/7 weeks, full term 39 0/7–40 6/7, late term 41 0/7–41 6/7, and post-term 42 0/7 and beyond; birth before 37 weeks is preterm.
Obstetric history alongside the EDD
Dating questions often come with an obstetric history. Gravida is the number of pregnancies (including the current one); para counts births after 20 weeks. TPAL breaks para into Term, Preterm, Abortions (losses before 20 weeks) and Living children. For example, G3P2002 is a woman in her third pregnancy with two term births, no preterm births, no losses and two living children. Decode any history with the gravida and para calculator or read gravida, para and TPAL explained.
Exam checklist
- Use the first day of the LMP.
- Add the days first, then deal with any month roll-over, then subtract 3 months.
- Don’t forget the year (+1, or −1 then +1 when you cross back over January).
- Write POG as completed weeks + days, never rounded up.
- Adjust for a long or short cycle if the question gives one.
- If an early scan date is given and differs beyond the threshold, the scan wins.
Want a hands-on aid for revision? Print the free paper OB wheel, or use the online OB wheel to check your answers.
Frequently asked questions
What is the formula for calculating EDD?
Naegele’s rule: EDD = first day of LMP + 7 days − 3 months + 1 year. It is equivalent to adding 9 months and 7 days, or about 280 days.
How do you calculate POG in weeks?
Count the days from the first day of the LMP to the date you need, divide by 7, and write it as completed weeks plus remaining days, such as 13+1.
What if the LMP is in January, February or March?
Subtracting 3 months takes you into the previous year, so the +1 year brings you back to the LMP’s year. Adding 9 months and 7 days directly gives the same EDD faster.
How do you adjust the EDD for a 35-day cycle?
Add the extra 7 days (35 − 28) to the Naegele EDD. For a shorter cycle, subtract the difference instead.
When is the EDD changed after an ultrasound?
Under ACOG guidance, when a first-trimester scan differs from LMP dating by more than 5 days before 9 weeks, or by more than 7 days from 9 weeks to 13 weeks 6 days.