Body Health CalculatorHow to Calculate Due Date from Last Menstrual Period
For regular 28-day cycles, add 280 days (40 weeks) to the first day of the last menstrual period. Irregular cycles or uncertainty about LMP date warrant first-trimester ultrasound dating, which ACOG treats as more reliable than LMP alone.
Last updated: September 2026
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Estimated due date versus delivery day
Estimated due date (EDD) is a 40-week anchor from the last menstrual period in regular cycles, not a promise of delivery on that calendar day. ACOG notes only a small fraction of births occur on the exact EDD; most happen between 38 and 41 weeks.
Gestational age for prenatal visits counts from LMP or from ultrasound dating when LMP is uncertain. Consumer calculators should label output as estimate and link to CDC week-by-week pregnancy education.
Medical decisions about induction or fetal testing use the EDD documented in the prenatal chart, especially after first-trimester ultrasound.
Naegele rule step by step
Identify the first day of the last menstrual period (LMP), not mid-cycle spotting. Add 280 days (40 weeks). Calendar shortcut: add one year, subtract three months, add seven days to LMP.
Example: LMP January 1 gives EDD October 8 in a non-leap year illustration. Write LMP on your calendar to avoid confusing implantation spotting with day one.
If you chart ovulation, you still use LMP for standard obstetric dating unless your clinician adjusts for known conception timing.
Adjusting for cycle length
Classic Naegele assumes a 28-day cycle with ovulation around day 14. Cycles of 32 or 35 days often ovulate later, which can make LMP-only EDD a few days early relative to conception.
Some calculators add days when cycle length exceeds 28 or subtract when shorter. Document the cycle length you entered when comparing to ultrasound reports.
Irregular cycles after hormonal contraception cessation may make LMP unreliable until ultrasound dating is available.
When to adjust or use ultrasound
Cycles longer or shorter than 28 days may shift ovulation. If ultrasound CRL dating differs from LMP by more than guideline thresholds, use the ultrasound EDD in your medical record.
IVF pregnancies use transfer date plus embryo age instead of LMP. Enter those parameters from your fertility clinic, not generic LMP calculators.
Update pregnancy apps after anatomy scan PDFs arrive so kick-count and leave planning match the clinic chart.
ACOG gestational age labels (summary)
| Label | Weeks days |
|---|---|
| Early term | 37w0d to 38w6d |
| Full term | 39w0d to 40w6d |
| Late term | 41w0d to 41w6d |
| Postterm | 42w0d and beyond |
Using EDD for planning and CDC milestones
EDD helps schedule anatomy scans, glucose screening, and Tdap timing per CDC pregnancy week-by-week pages. It does not authorize early elective delivery before medical indications.
Work leave and childcare plans should include a window of several weeks around EDD rather than a single date.
Twins and higher-order multiples often deliver earlier; follow maternal-fetal medicine dating instead of singleton calculators alone.
Common LMP dating mistakes
Using the date of a positive pregnancy test instead of LMP start shifts EDD by weeks. LMP is the first day of bleeding before conception, confirmed with your clinician.
Ignoring cycle length when ovulation is late makes EDD several days early on calendars; note cycle length on the form you bring to the first obstetric visit.
Forgetting to update apps after ultrasound replaces LMP dating leaves kick-count reminders on the wrong week.
Confusing gestational age with fetal age (two weeks younger) misaligns CDC milestone articles; obstetric gestational age includes the pre-conception two weeks in standard dating.
Share EDD with partners and employers
Print EDD plus a two-week window for childcare and leave paperwork rather than a single calendar date.
Give doulas and pediatricians the ultrasound-dated EDD from your chart, not only the first app calculation from LMP.
Update workplace HR forms after anatomy scan revisions so short-term disability timing matches medical records.
CDC week-by-week links help partners understand why gestational week numbers differ from "months pregnant" casual language.
First trimester confirmation steps
Schedule dating ultrasound when LMP is unknown or cycles are irregular; ACOG pathways prioritize early biometry when LMP disagrees.
Bring prior pregnancy records if this is a subsequent pregnancy; LMP memory errors are common on intake forms.
Note bleeding dates separately from LMP until your obstetric clinician confirms which day starts gestational counting.
Paper versus app calculators
Paper Naegele worksheets help when you lack signal at early prenatal visits; transfer the result into your portal when connectivity returns.
Apps should display LMP, cycle length, and resulting EDD on one screen for audit by your clinician.
Set reminders for anatomy scan weeks based on ultrasound-confirmed EDD, not the first guess from memory alone.
Export calendar files in UTC-neutral local dates to avoid off-by-one EDD errors when syncing across time zones.
FAQs
Can I use implantation bleeding as LMP?
No. LMP is the first day of true menstrual flow before pregnancy, confirmed with your clinician.
Should apps update after ultrasound?
Yes when ultrasound dating replaces LMP in your medical record.
Does EDD predict exact delivery?
No. Most births occur in a multi-week window centered near 40 weeks.