Expected Date of Delivery (EDD)
The Expected Date of Delivery (EDD) represents the estimated date when childbirth is likely to occur. It is calculated from the first day of the Last Menstrual Period (LMP) using Naegele’s Rule.
Naegele’s Rule
For women with regular 28-day menstrual cycles:
EDD=LMP+9 months+7 days
This formula assumes that:
-The ovulation occurs on the 14th day of the cycle and fertilization soon after and
-That the AN woman has a regular 28-day menstrual cycle
Conditions for Reliable Use of Naegele’s rule: This rule provides accurate estimation only when:
• The woman has regular 28 menstrual cycles before conception.
• There is no recent use of hormonal contraceptives, as ovulation may be delayed for 2–3 weeks after stopping oral contraceptive pills (OCPs).
Fertilization or Ovulatory Age
The fertilization (ovulatory) age is approximately two weeks shorter than the gestational age calculated from the LMP. This distinction is particularly relevant in assisted reproductive techniques (ART) such as IVF or IUI, where the exact date of fertilization is known.
Cycle Length Correction to Naegele’s Rule
When the menstrual cycle length differs from the standard 28 days, EDD should be adjusted accordingly:
• If cycle length > 28 days: Add the extra days to the EDD.
Example: For a 35-day cycle → add 7 extra days
Corrected EDD = LMP + 9 months + 7 days + 7 days
Corrected EDD=LMP+9 months+7 days+7 days
• If cycle length < 28 days: Subtract the difference.
Example: For a 25-day cycle → subtract 3 days
Corrected EDD = LMP + 9 months + 7 days – 3 days
Corrected EDD=LMP+9 months+7 days-3 days
• If cycles are irregular or > 45 days: The LMP is unreliable. In such cases, EDD and POG should be determined through dating ultrasound and clinical examination findings.
Accuracy of EDD
Only about 4% of women deliver exactly on the estimated date. Most give birth within two weeks before or one week after the EDD.
When the LMP is Unknown
If the woman does not recall her LMP, the probable date of conception or gestation can be estimated from:
1. History
• Ask the woman to relate her LMP to festivals or personal events she remembers.
• Urine Pregnancy Test (UPT): Usually becomes positive 11–15 days after conception, or 1–2 days after a missed period in a regular 28-day cycle.
• Quickening (first perception of fetal movement):
Multipara—16–18 weeks;
Primigravida—18–20 weeks.
• Infertility treatment records (IUI/IVF) provide exact conception dates.
2. Obstetric Examination
Review early antenatal records for uterine size and fundal height measurements to approximate gestational age.
3. Ultrasound Examination
A first-trimester ultrasound (CRL measurement) is the most accurate for dating pregnancy.
Fetal heart rate (FHR) detection:
• Transabdominal ultrasound – 7 weeks
• Handheld Doppler – 10 weeks
• Stethoscope (Pinard’s) – 20 weeks
Postnatal Assessment
When gestational age remains uncertain, neonatal maturity can be assessed after birth using Ballard’s scoring system, which evaluates physical and neuromuscular maturity of the newborn.
References
Dutta DC. Textbook of Obstetrics. 9th ed. New Delhi: Jaypee Brothers Medical Publishers; 2023.
Padubidri VG, Daftary SN. Shaw’s Textbook of Gynaecology. 18th ed. New Delhi: Elsevier; 2022.
Park K. Park’s Textbook of Preventive and Social Medicine. 28th ed. Jabalpur: Bhanot Publishers; 2025.
Babu SA. Clinical Obstetrics and Gynecology. 2nd ed. New Delhi: Wolters Kluwer India Pvt Ltd; 2025.
Obstetric index (GPAL) in Antenatal Case History: https://ihatepsm.com/blog/obstetric-index-gpal-antenatal-case-history
Decoding Gravida and Para: Terms in Antenatal History Taking: https://ihatepsm.com/blog/decoding-gravida-and-para-terms-antenatal-hist...
Duration of Pregnancy: Understanding the Trimesters and Gestational Age Categories: https://ihatepsm.com/blog/duration-pregnancy-understanding-trimesters-an...
Calculation of Expected Date of Delivery (EDD) and Period of Gestation (POG): https://ihatepsm.com/blog/calculation-expected-date-delivery-edd-and-per...