Estimated Date of Birth Calculator
1Method of conception
280 days from LMP, adjusted by one day for each day the cycle differs from 28. The adjustment assumes the variation sits in the follicular phase.
For a woman with no usable LMP and no dating ultrasound. Estimated date of birth is 266 days from ovulation. An intercourse date is not a conception date and should not be entered here.
Practice points, timing and accuracy
The calculation
Estimated date of birth is the ovulation date plus 266 days, which is 38+0. The same date comes from treating the ovulation date minus 14 days as a certain LMP and applying Naegele's rule. Documenting it that way keeps the record in the LMP-referenced framework that screening windows, biometry charts and post-dates thresholds all assume. Current gestation is days since ovulation, plus 14.
This is the logic ACOG applies to ART, where gestational age derived from a known conception date is used to assign the date and is not overridden by ultrasound. A well-documented natural ovulation date sits on the same footing: LMP error is overwhelmingly ovulation-timing error, so removing that step removes the main source of imprecision.
How certain is the date
- IVF or ICSI retrieval or transfer. Effectively exact.
- IUI, or a timed cycle with an hCG trigger. Around one day.
- Serial transvaginal follicular tracking. Around one day.
- Urinary LH kit. The kit marks the surge, not ovulation. Ovulation follows roughly 24 to 36 hours after surge onset, so add a day if she is dating from the first positive.
- Basal body temperature shift. Retrospective and soft, around two days.
- Cervical mucus peak day. The weakest of these. Peak day approximates ovulation within one to two days and is subjective.
The ovum is fertilisable for only 12 to 24 hours after ovulation, so the ovulation date can be read as the conception date. An intercourse date cannot, because sperm survive up to five days.
What the research says
266 days is a convention, not a biological midpoint. In the Jukic cohort of 125 naturally conceived singletons, with ovulation identified by daily urinary hormone assay, median time from ovulation to birth was 268 days, or 38+2. A conception-derived date therefore sits about two days early against observed spontaneous onset. Keep 266 as the clinical date. Do not run two dates in the record.
That same cohort spanned 37 days once preterm births were excluded, which is worth saying to a woman anxious about her date: even exact dating does not narrow the birth window much. Dating by ovulation is still the more precise of the two, with a coefficient of variation of 3.7 per cent against 4.9 per cent by LMP.
Where no ultrasound is available
RANZCOG and the Australian Pregnancy Care Guidelines position first-trimester ultrasound as the reference standard, and ACOG regards a pregnancy not confirmed or revised by ultrasound before 22+0 as suboptimally dated. If she declines a scan, document the discussion, her reasons, and the downstream implications for screening windows and post-dates surveillance.
If a scan is done later and disagrees, a certain conception date is not redated unless the difference exceeds the threshold for that gestation. A difference inside the threshold is more likely growth variation than a dating error. Document the date and its basis, and reserve later changes for rare circumstances.
References
ACOG, AIUM and SMFM. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol 2017;129:e150-4. PMID 28426621. acog.org
Jukic AM, Baird DD, Weinberg CR, McConnaughey DR, Wilcox AJ. Length of human pregnancy and contributors to its natural variation. Hum Reprod 2013;28(10):2848-55. PMID 23922246. academic.oup.com
Butt K, Lim KI. SOGC Guideline No. 388: Determination of Gestational Age by Ultrasound. J Obstet Gynaecol Can 2019.
ASUM. Guidelines for the Performance of First Trimester Ultrasound.
Australian Government Department of Health. Pregnancy Care Guidelines.
Estimated date of birth is 266 days from this date.
Transfer date plus 266 days, less the age of the embryo at transfer. Applies to both fresh and frozen cycles. IVF dating stands; ultrasound does not override it.
2Ultrasound (optional)
Robinson–Fleming dating. Applies between 10 and 84 mm.
Use the gestation stated on the report.
3Which estimated date of birth to use
Enter a date to begin.
weeks
Defaults to today. Change it to date a retrospective note or to look ahead.
- Estimated date of birth in use
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- Trimester
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- Days to estimated date of birth
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- Reaches 37 weeks
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- Reaches 42 weeks
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- CRL dating window (8+0 to 13+6)
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Key dates across the pregnancy
Schedule prompts only. Adjust to your own care pathway.
This is a calculation aid only. Dating decisions, discrepancy thresholds and escalation follow your clinical judgement. All dates should be reviewed for accuracy.