How accurate is the due date calculated from LMP?

LMP-based due date is accurate within ±7–10 days in women with regular 28-day cycles. A 1st trimester ultrasound reduces error to ±5 days.

What is full term pregnancy?

Full term is between 39 weeks 0 days and 40 weeks 6 days. Delivery before 37 weeks is considered preterm.

Medically Reviewed · CliniqWise Clinical Advisory Board — Obstetrics & Gynecology Panel · Updated 2026-08-27

Pregnancy Due Date Calculator — Estimated Date of Delivery (EDD) & Gestational Age

Calculate estimated date of delivery (EDD) and current gestational age from Last Menstrual Period (LMP) using Naegele’s Rule or conception date.

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Pregnancy Due Date Calculator

Enter values below to calculate

Days elapsed since first day of LMP*

days

Number of days from 1st day of last period to today (e.g. 70 days = 10 weeks).

Average Menstrual Cycle Length*

days

Standard is 28 days.

This calculator is for informational and educational purposes. Results should be interpreted with clinical judgment and local guidelines.

What is Naegele’s Rule for Pregnancy Due Date?

Naegele’s Rule is a standard clinical method for estimating the expected date of delivery (EDD, or estimated date of confinement [EDC]) based on the first day of the patient’s last menstrual period (LMP). Formulated by German obstetrician Franz Karl Naegele in the 19th century, the formula adds 280 days (40 weeks, or 9 calendar months plus 7 days) to the first day of the LMP, assuming a standard 28-day menstrual cycle with ovulation occurring on day 14. If cycle length differs from 28 days, an adjustment is made by adding or subtracting the difference in days. First-trimester crown-rump length (CRL) ultrasound remains the most accurate method for confirming or revising gestational age.

Naegele’s Rule: EDD = First Day of LMP + 1 Year − 3 Months + 7 Days. Standard human gestation = 280 days (40 weeks) from LMP.

When to Use the Pregnancy Due Date Calculator

Initial prenatal consultation and dating of pregnancy in obstetrics and primary care.
Scheduling key prenatal screening tests (nuchal translucency at 11–13+6 weeks; anomaly scan at 18–22 weeks; glucose tolerance test at 24–28 weeks).
Determining gestational age for viability, preterm labor management, and post-dates induction of labor planning (41–42 weeks).

Pregnancy DatingAntenatal Care SchedulingPreterm Labor Risk AssessmentPost-term / Post-dates ManagementObstetric Ultrasound Correlation

Naegele’s Formula & Cycle Adjustment

Naegele’s Standard Rule (28-day cycle)

EDD = LMP + 7 Days − 3 Months + 1 Year (or LMP + 280 Days)

LMP = January 15 → EDD = October 22 (Jan 15 + 7 days = Jan 22; Jan − 3 months = October)

Cycle Length Adjusted Rule

EDD = LMP + 280 Days + (Cycle Length in days − 28)

LMP = Jan 15, 32-day cycle → EDD = Oct 22 + (32 − 28) = October 26

Variables

Last Menstrual Period (LMP) (date)

First day of the last normal menstrual period.

Average Cycle Length (days)

Normal interval between periods. Standard is 28 days (ovulation at day 14).

If early 1st trimester ultrasound (CRL) differs by > 5–7 days from LMP EDD, the ultrasound EDD is preferred per ACOG guidelines.

Pregnancy Trimester Timeline & Milestones

Gestational age progress based on 40 weeks total duration.

Range	Classification	Health Risk
1–13 Weeks	First Trimester	Organogenesis. Viability scan, NIPT / Nuchal translucency screen.
14–27 Weeks	Second Trimester	Rapid growth. Anomaly scan (18–22w), OGTT glucose screening (24–28w).
28–36 Weeks	Third Trimester (Preterm)	Fetal maturation. Group B Strep screen (35–37w), growth scans.
37–41 Weeks	Term Pregnancy	Early term (37–38w), Full term (39–40w), Late term (41w). Spontaneous labor window.
≥ 42 Weeks	Post-term Pregnancy	Increased stillbirth risk. Induction of labor indicated per ACOG/RCOG.

Full term is defined as 39 weeks 0 days through 40 weeks 6 days.

Limitations & Clinical Warnings

Assumes regular menstrual cycles; inaccurate in irregular cycles, post-oral contraceptive pill withdrawal, or breastfeeding amenorrhea.

Only 4–5% of women deliver on their exact estimated due date; 90% deliver within ±2 weeks of EDD.

Ultrasound dating in 1st trimester (CRL) takes precedence over LMP when dates conflict per ACOG Committee Opinion No. 700.

Medical Disclaimer

Calculated EDD is an estimate. Clinical management must integrate ultrasound imaging and clinical examination.

References & Citations

ACOG, AIUM, SMFM. “Methods for Estimating the Due Date. Committee Opinion No. 700”. Obstetrics & Gynecology. 2017. 130(5): e229–e232. PMID: 28937572 | DOI: 10.1097/AOG.0000000000002355

Medically Reviewed

Reviewed by CliniqWise Clinical Advisory Board — Obstetrics & Gynecology Panel · Last updated: 2026-08-27 · Next review: 2027-02-27

Frequently Asked Questions

Common Questions About Pregnancy Due Date Calculator

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