LMP stands for the first day of your last menstrual period. Standard obstetric practice counts gestational age from this date even though conception happens roughly two weeks later. Using LMP as the starting point gives a consistent reference that is easy to recall and aligns with the 40-week pregnancy timeline most providers and pregnancy apps follow.

How does cycle length affect the due date?

Standard gestational calculations assume a 28-day cycle with ovulation on day 14. If your cycle is consistently longer or shorter than 28 days, ovulation and conception likely occurred earlier or later than that assumption. The estimated due date shifts by roughly the same number of days as your cycle differs from 28. Gestational age is still tracked from the adjusted LMP reference date.

Should I use conception date or ultrasound?

A first-trimester ultrasound, typically performed between 8 and 13 weeks, provides the most accurate dating because it measures fetal size directly and is less dependent on cycle regularity. Use LMP or conception date as an initial estimate when a dating scan is not yet available. If your ultrasound and LMP dates differ by more than a week, most providers will use the scan date.

How accurate are these estimates?

Due date calculations are estimates, not precise predictions. Only a small percentage of births occur exactly on the estimated due date. Individual variation in ovulation timing, implantation, and fetal growth means the actual birth can reasonably fall two weeks before or after the estimate. Always confirm your due date with your healthcare provider, especially if cycles are irregular.

What if my estimated due date has already passed?

If the calculated date falls before today, the calculator will flag it as past. A due date passing without delivery, called a post-term pregnancy, is a clinical situation that requires monitoring. Providers typically assess the situation with non-stress tests or biophysical profiles and discuss options with you. Contact your healthcare provider promptly for guidance if this applies to you.

Pregnancy Due Date Calculator

Estimate due date and current gestational age with a trimester timeline.

Method

First day of LMPAverage cycle length (days)

Due date

Feb 14, 2027

Due in 32 weeks 0 days.

Current gestational age

8 weeks 0 days

Trimester: 1

Trimester progress

Based on an estimated 40-week pregnancy timeline.

Milestone timeline

Heart detectable (week 6)
Gestational age: 6 weeks 0 days
Jun 21, 2026
First trimester end (week 13)
Gestational age: 13 weeks 0 days
Aug 09, 2026
Anatomy scan (week 19-20)
Gestational age: 19 weeks 0 days
Sep 20, 2026
Glucose test (week 24-28)
Gestational age: 25 weeks 0 days
Nov 01, 2026
Third trimester begins (week 28)
Gestational age: 28 weeks 0 days
Nov 22, 2026
Term (week 37)
Gestational age: 37 weeks 0 days
Jan 24, 2027
Estimated due date (week 40)
Gestational age: 40 weeks 0 days
Feb 14, 2027

This tool uses standard obstetric formulas. Always confirm your due date with your healthcare provider, often using an ultrasound.

How this tool works

The calculator supports three estimation methods. Naegele's Rule (LMP): due date = last menstrual period date + 280 days. If your cycle differs from 28 days, the tool adds (cycle length − 28) days to adjust, because ovulation and conception shift proportionally. Conception date method: adds 266 days (38 weeks) directly to the confirmed or estimated conception date, bypassing the LMP-to-conception lag. Ultrasound method: adds the difference between 280 days and the gestational age recorded at your scan (converted to days) to the scan date, back-calculating the corrected due date from the ultrasound measurement. All three methods then calculate current gestational age in weeks and days and project trimester milestones. Key assumption: Naegele's Rule assumes a 28-day cycle and ovulation on day 14; actual ovulation timing varies. Edge case: the 280-day standard targets a singleton pregnancy; twin and higher-order pregnancies have a median delivery date of 36–37 weeks and 34–35 weeks respectively, so the estimated due date is not clinically applicable without medical guidance.

Worked example

Example: If your last menstrual period started 8 weeks ago and your cycle length is 28 days, your current gestational age is about 8 weeks and your estimated due date is about 40 weeks from LMP. If you enter a 35-day cycle length, the estimated due date shifts later by about 7 days compared with a 28-day cycle.

Frequently asked questions

What is LMP?
LMP stands for the first day of your last menstrual period. Standard obstetric practice counts gestational age from this date even though conception happens roughly two weeks later. Using LMP as the starting point gives a consistent reference that is easy to recall and aligns with the 40-week pregnancy timeline most providers and pregnancy apps follow.
How does cycle length affect the due date?
Standard gestational calculations assume a 28-day cycle with ovulation on day 14. If your cycle is consistently longer or shorter than 28 days, ovulation and conception likely occurred earlier or later than that assumption. The estimated due date shifts by roughly the same number of days as your cycle differs from 28. Gestational age is still tracked from the adjusted LMP reference date.
Should I use conception date or ultrasound?
A first-trimester ultrasound, typically performed between 8 and 13 weeks, provides the most accurate dating because it measures fetal size directly and is less dependent on cycle regularity. Use LMP or conception date as an initial estimate when a dating scan is not yet available. If your ultrasound and LMP dates differ by more than a week, most providers will use the scan date.
How accurate are these estimates?
Due date calculations are estimates, not precise predictions. Only a small percentage of births occur exactly on the estimated due date. Individual variation in ovulation timing, implantation, and fetal growth means the actual birth can reasonably fall two weeks before or after the estimate. Always confirm your due date with your healthcare provider, especially if cycles are irregular.
What if my estimated due date has already passed?
If the calculated date falls before today, the calculator will flag it as past. A due date passing without delivery, called a post-term pregnancy, is a clinical situation that requires monitoring. Providers typically assess the situation with non-stress tests or biophysical profiles and discuss options with you. Contact your healthcare provider promptly for guidance if this applies to you.