Naegele’s rule: the LMP-based standard
The most widely used method for estimating a due date is Naegele’s rule, named after the 19th-century German obstetrician Franz Karl Naegele. The rule adds 280 days (40 weeks) to the first day of the last menstrual period (LMP). Those 280 days represent an average pregnancy duration of 40 weeks measured from the LMP, which is itself roughly 14 days before ovulation in a 28-day cycle. The result is called the estimated date of delivery (EDD).
Because Naegele’s rule assumes a 28-day cycle with ovulation on day 14, calculators often apply a cycle-length correction: for each day the cycle exceeds 28 days, one day is added to the EDD, and for each day it is shorter, one day is subtracted. This adjustment helps align the LMP-based estimate more closely with the biological event of conception.
ACOG Committee Opinion 700 (2017) describes Naegele’s rule as the starting point for gestational age dating, while also noting that ultrasound measurement — particularly in the first trimester — is more precise when there is any uncertainty about the LMP date or cycle regularity.
| Dating method | Input used | Days added | Based on |
|---|---|---|---|
| Naegele’s rule (LMP, 28-day cycle) | First day of last menstrual period | 280 | Average gestational length from LMP |
| Conception dating | Known or estimated date of conception | 266 | Average embryonic developmental period |
| IVF day-3 transfer | Date of embryo transfer | 263 | Gestational age at transfer = 17 days |
| IVF day-5 transfer (blastocyst) | Date of embryo transfer | 261 | Gestational age at transfer = 19 days |
Conception dating: when the day of conception is known
When a person knows their approximate date of conception — for example, because they tracked ovulation with LH tests or basal body temperature — the EDD can be estimated by adding 266 days. This figure reflects the average duration of human embryonic and fetal development from fertilization to birth. It is equivalent to the 280-day Naegele estimate minus the roughly 14 days between LMP and ovulation in a standard cycle.
Conception dating is also used when the LMP is unknown, for instance after a period of irregular cycles or when a person was not tracking their cycle. In these situations, a clinician will typically confirm or revise the estimate with an ultrasound measurement of crown-rump length (CRL) during the first trimester.
IVF dating: day-3 and day-5 embryo transfers
In pregnancies achieved through in vitro fertilization (IVF), the date of embryo transfer is precisely known, making gestational age calculation more straightforward. For a day-3 transfer, the embryo is considered to be 17 days past the LMP equivalent (14 days to ovulation plus 3 days of in-vitro development), so 263 days are added to the transfer date to estimate the EDD. For a day-5 blastocyst transfer, 19 days are subtracted from the 280-day standard, leaving 261 days added to the transfer date.
These IVF-specific offsets are consistent with the dating conventions described in ACOG Committee Opinion 700 and are used by most fertility clinics. An OB/GYN or reproductive endocrinologist will confirm gestational age at the first obstetric ultrasound, which typically occurs around 8–10 weeks.
First-trimester ultrasound: the most accurate dating tool
ACOG Committee Opinion 700 states that ultrasound measurement of the embryonic crown-rump length (CRL) between approximately 8 weeks 0 days and 13 weeks 6 days is the most accurate method for confirming or revising gestational age. At this stage, CRL can estimate gestational age to within approximately plus or minus 5–7 days. After 14 weeks, measurements of the biparietal diameter (BPD) and other fetal biometrics are less precise, with accuracy declining as pregnancy advances.
When the ultrasound-derived EDD differs from the LMP-based EDD by more than the threshold specified in ACOG guidelines (5–7 days in the first trimester, larger thresholds in the second and third trimesters), ACOG recommends redating the pregnancy using the ultrasound estimate. This practice reduces unnecessary interventions related to apparent post-term pregnancy.
Why babies rarely arrive exactly on the due date
The EDD is an estimate, not a prediction. Published cohort data show that spontaneous onset of labor is distributed across a range of weeks around the due date, and that only a minority of births occur on the exact day estimated. ACOG uses the term “full term” to describe deliveries between 39 weeks 0 days and 40 weeks 6 days, and recognizes that normal uncomplicated pregnancies may result in birth anywhere from 37 to 42 weeks.
Several biological variables contribute to this variability: natural differences in gestational length among individuals, the inherent imprecision of LMP recall, cycle-length variation, and the statistical nature of any population average. Prenatal care providers use the EDD as a clinical reference point for scheduling screenings and monitoring fetal growth, not as a firm deadline. Discussions about management of pregnancies that extend beyond 41 or 42 weeks should take place with an OB/GYN or midwife.
Using a due date calculator
Online due date calculators apply the same arithmetic as Naegele’s rule and the IVF offsets described above. They are educational tools that provide an estimate based on the dates entered. Any calculator result should be reviewed with a qualified prenatal care provider who can confirm gestational age with an ultrasound and account for individual clinical factors.
The pregnancy due date calculator on Calculate.Studio accepts LMP, conception date, IVF day-3, and IVF day-5 inputs. It also adjusts for self-reported cycle length when the LMP method is selected. Results display the estimated due date along with approximate trimester-start dates based on the 280-day reference timeline.
अक्सर पूछे जाने वाले सवाल
What is Naegele’s rule?
Naegele’s rule is the standard formula for estimating a pregnancy due date. It adds 280 days (40 weeks) to the first day of the last menstrual period (LMP). The rule assumes a 28-day cycle; calculators often apply a correction for longer or shorter cycles.
How many days are added for an IVF day-5 transfer?
For a day-5 blastocyst transfer, 261 days are added to the embryo transfer date to estimate the due date. This accounts for the 19 days of in-vitro development (14 days to ovulation plus 5 days in culture) that precede the transfer.
Is a first-trimester ultrasound more accurate than LMP dating?
Yes. ACOG Committee Opinion 700 (2017) states that ultrasound measurement of crown-rump length between approximately 8 and 14 weeks provides the most accurate estimate of gestational age, generally within plus or minus 5–7 days. LMP-based dating can be less precise if the LMP date is uncertain or if cycles are irregular.
Do most babies arrive on their due date?
No. Published cohort research shows that spontaneous birth on the exact estimated due date is uncommon. ACOG considers the period from 39 weeks 0 days to 40 weeks 6 days as “full term,” and normal deliveries may occur anywhere from 37 to 42 weeks. The due date is an estimate, not a guaranteed delivery date.
What is the due date when conception date is known?
When the date of conception is known, adding 266 days gives the estimated due date. This is equivalent to the 280-day LMP estimate minus the approximately 14 days between the LMP and ovulation in a standard 28-day cycle.
When should the due date be revised based on ultrasound?
According to ACOG Committee Opinion 700, the due date should be revised if the first-trimester ultrasound estimate differs from the LMP-based estimate by more than 5–7 days (specific thresholds vary by trimester). Clinicians make this determination during prenatal care.
संदर्भ
- American College of Obstetricians and Gynecologists (ACOG). Committee Opinion 700: Methods for Estimating the Due Date. Obstetrics & Gynecology. 2017;129(5):e150–e154.
- Naegele FC. Erfahrungen und Abhandlungen aus dem Gebiete der Krankheiten des weiblichen Geschlechtes. 1812. Mannheim: Schwan und Götz. [Historical primary source establishing the rule.]
- American College of Obstetricians and Gynecologists (ACOG). Definition of Term Pregnancy. Committee Opinion 579. Obstetrics & Gynecology. 2013;122(5):1139–1140.
- Savitz DA, Terry JW Jr, Dole N, Thorp JM Jr, Siega-Riz AM, Herring AH. Comparison of pregnancy dating by last menstrual period, ultrasound scanning, and their combination. American Journal of Obstetrics and Gynecology. 2002;187(6):1660–1666.
- Jukic AM, Baird DD, Weinberg CR, McConnaughey DR, Wilcox AJ. Length of human pregnancy and contributors to its natural variation. Human Reproduction. 2013;28(10):2848–2855.
- National Health Service (NHS). Your pregnancy due date. Available at: www.nhs.uk. Accessed 2026.