# Due Date Calculator Alternatives: LMP and Ultrasound Dating

> Due date estimators apply Naegele rule (LMP + 280 days) or adjust from ultrasound crown-rump length in the first trimester. First-trimester ultrasound dating is more accurate than LMP alone when cycles are irregular.

**Last updated:** September 2026
**URL:** https://bodyhealthcalculator.com/alternatives/due-date-calculator

## Pregnancy dating tools

Due date calculator alternatives include LMP-based Naegele rule apps, ovulation and conception date estimators, first-trimester ultrasound biometry calculators used in clinics, IVF embryo transfer date math, and paper obstetric wheels still found in prenatal offices.

Estimated due date (EDD) is a 40-week projection, not a delivery appointment. ACOG notes only a small fraction of births occur on the exact EDD; most deliver between 38 and 41 weeks.

Accurate dating matters for timing of anatomy scans, gestational diabetes screening, and decisions about induction when pregnancies continue past term.

Consumer apps add kick counters and weekly size comparisons, but the legal chart date usually comes from the clinic ultrasound report or documented LMP.

*Pregnancy dating methods*

| Method | Inputs | Accuracy | When to prefer |
| --- | --- | --- | --- |
| Naegele rule (LMP + 280 days) | First day of LMP | Assumes 28-day cycles | Regular cycles, sure LMP |
| Ultrasound CRL (first trimester) | Crown-rump length | ±5 to 7 days | Unknown LMP or irregular cycles |
| Conception date + 266 days | Known conception/IUI | Good if date certain | Assisted reproduction logs |
| IVF transfer calculator | Transfer day + embryo age | High when documented | IVF pregnancies |
| Second-trimester ultrasound | Biparietal diameter etc. | Less accurate than first | Late presentation |
| Physical exam (fundal height) | Weeks gestation estimate | Screening only | Not for precise EDD |

Sources: [1]

## LMP and Naegele rule

Add one year, subtract three months, add seven days to the first day of the last menstrual period (or simply add 280 days) for a 28-day cycle assumption. Longer or shorter cycles need adjustment or ultrasound dating.

If the person uses hormonal contraception recently or has bleeding that is not a true period, LMP dating can be wrong by weeks. First-trimester ultrasound should reassign EDD when discrepancy exceeds guideline thresholds.

CDC pregnancy week-by-week resources count gestational age from LMP even though fertilization typically occurs about two weeks later. That convention keeps prenatal visits aligned with obstetric charts.

Write down whether LMP was certain before you share a free calculator result with your prenatal team.

Sources: [1] [3]

## Ultrasound trumping LMP

ACOG supports ultrasound measurement of crown-rump length between 7 and 13 weeks as the most accurate biometric dating. When ultrasound EDD differs from LMP-based EDD beyond specified day cutoffs, clinical records should use the ultrasound date.

Patients can still use free LMP calculators for a preliminary date before the first scan, then update the profile after the sonographer report.

Before 9 weeks gestation, a discrepancy of more than five days between LMP and CRL dating may warrant changing the EDD. Cutoffs widen slightly as pregnancy progresses.

Mean sac diameter alone is not recommended for dating when CRL is available because error is larger.

Sources: [2] [3]

## IVF, IUI, and known conception dates

Embryo transfer calculators add gestational age based on embryo day plus transfer date, producing EDD without relying on LMP at all when documentation is complete.

Known ovulation or intrauterine insemination dates use conception plus about 266 days instead of 280 days from LMP, reflecting the two-week difference between LMP-based and fertilization-based counting.

ACOG guidance still recommends ultrasound confirmation even in assisted reproduction when early scans are available, because implantation timing can vary.

Keep clinic embryo records as the authoritative source when they disagree with a consumer app by more than a few days.

Sources: [1] [2]

## Patient portals vs standalone calculators

Hospital portals pull EDD from obstetric triage and ultrasound reports, which is the legal record for induction timing discussions. Consumer apps add weekly fruit-size comparisons and kick counters; verify that the underlying date matches your clinician chart.

Reverse due date calculators work backward from a desired delivery window for planning maternity leave; they still depend on the same LMP or ultrasound anchor.

Suboptimally dated pregnancies without ultrasound before 22 weeks may need later scans interpreted cautiously for growth, not just for entertainment milestones.

Twins share one dating anchor but often deliver earlier; follow maternal-fetal medicine guidance rather than singleton app reminders alone.

Sources: [1] [4]

## Gestational age labels beyond the EDD

CDC preterm birth resources define preterm as before 37 completed weeks. Early term, full term, and late term windows help parents interpret why induction might be offered near 41 weeks.

EDD shifts should be rare after the first trimester ultrasound is accepted. Document any change in the portal and notify your birth team.

Fundal height measurements during prenatal visits screen for growth issues but do not replace ultrasound dating or biometry.

If your calculator shows 40 weeks on a day your clinic lists 39, reconcile the anchor date before you plan travel or leave from work.

International travel near term should include printed ultrasound summaries because gestational age labels differ from casual app wording.

*Selected gestational age milestones (clinical context)*

| Label | Approximate weeks |
| --- | --- |
| Early preterm | Before 34 weeks |
| Late preterm | 34 to 36 weeks |
| Early term | 37 to 38 weeks |
| Full term | 39 to 40 weeks |

Sources: [3] [4]

## Keeping one official EDD across tools

Screenshot or export portal dates after each ultrasound so consumer apps can be edited to match.

When employers ask for due dates, use the clinician chart EDD rather than a forward-looking reverse calculator meant for leave planning.

If LMP is updated after chart review, rerun only the tools that allow manual override instead of trusting cached app profiles.

Birth plans and pediatrician selection can proceed using week ranges, but medical decisions always follow the documented gestational age in the prenatal record.

Sources: [1] [3]

## FAQ

### Can my due date change after ultrasound?

Yes. First-trimester biometry often replaces LMP dating when they disagree beyond guideline limits.

### Is conception date nine months before due date?

Pregnancy is counted from LMP, about two weeks before ovulation. Conception-based math uses roughly 266 days from fertilization, not 280.

### Do twins share one due date?

Twins are dated with the same LMP or ultrasound anchor, but delivery timing and risks differ. Follow maternal-fetal medicine guidance.

## References

1. ACOG. [Methods for estimating due date (ACOG)](https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date)
2. ACOG. [Ultrasound dating criteria (ACOG reVITALize)](https://www.acog.org/clinical/clinical-guidance/obstetric-care-consensus/articles/2016/10/perinatal-quality-collaborative)
3. CDC. [Pregnancy week by week (CDC)](https://www.cdc.gov/pregnancy/week-by-week/index.html)
4. CDC. [Preterm birth and gestational age (CDC)](https://www.cdc.gov/maternal-infant-health/preterm-birth/index.html)

## Related calculators

- [gestational-age-calculator](https://bodyhealthcalculator.com/gestational-age-calculator.md)
- [conception-calculator](https://bodyhealthcalculator.com/conception-calculator.md)
- [ovulation-calculator](https://bodyhealthcalculator.com/ovulation-calculator.md)
- [pregnancy-weight-gain-calculator](https://bodyhealthcalculator.com/pregnancy-weight-gain-calculator.md)