Pregnancy Due Date Calculator
Estimate your expected due date using the first day of your last menstrual period (LMP) and your usual menstrual cycle length. The calculator also shows estimated gestational age, trimester, pregnancy progress and an approximate conception date.
Get an estimated due date in three steps
You will need the first day of your last menstrual period and, if you know it, your usual cycle length. If your periods are irregular or you are unsure of your LMP date, the result may be less reliable.
Enter your LMP
Select the first day of your last menstrual period.
Choose cycle length
Select the number of days in your usual menstrual cycle.
View your estimate
See your estimated due date and pregnancy timeline.
Calculate Your Estimated Due Date
Enter the first day of your last menstrual period (LMP) and your usual cycle length. The result is an estimate and should not replace pregnancy dating by a healthcare professional.
How the Due Date Estimate Works
A commonly used starting point for pregnancy dating is the first day of the last menstrual period. For a 28-day cycle, the estimated due date is about 280 days (40 weeks) from that date. This calculator shifts the estimate when the selected cycle length differs from 28 days. Because ovulation timing and cycle patterns vary, the result should be treated as an estimate rather than an exact prediction.
First Trimester
The first trimester covers the early stage of pregnancy. Prenatal care and accurate pregnancy dating are especially useful during this period.
Second & Third Trimesters
Pregnancy progress is usually followed in weeks. Your clinician may use ultrasound and other clinical information when confirming or revising dating.
Important Medical Note
What does average cycle length mean?
Cycle length is counted from the first day of one menstrual period to the first day of the next. Many due-date calculations use a 28-day cycle as a reference, but real menstrual cycles can be shorter or longer.
If your usual cycle length differs from 28 days, selecting the closest typical length can make the estimate more personalized. This still cannot identify your exact ovulation or conception date.
Start day
Day 1 is the first day of menstrual bleeding.
Next period
Count through the day before the next period starts.
Use your usual length
If cycles vary, remember the calculator can only provide an approximation.
What the calculator can — and cannot — tell you
Estimated due date
A calendar estimate based on the LMP date and selected cycle length. It is not a guaranteed delivery date.
Gestational age
An estimate of how far along the pregnancy is, counted from the first day of the LMP rather than from the date of conception.
Approximate conception date
A rough estimate only. Ovulation and conception do not occur on the same cycle day for everyone.
Understand the pregnancy timeline by trimester
Pregnancy is usually tracked in completed weeks of gestation. On this website, the trimester labels used by the calculator are: first trimester through 13 weeks, second trimester from 14 through 27 weeks, and third trimester from 28 weeks onward.
First Trimester
Weeks 1–13.
Second Trimester
Weeks 14–27.
Third Trimester
Week 28 onward.
When a due-date estimate may be less reliable
An LMP-based calculator is convenient, but it has limits. The estimate can be less dependable when the LMP date is uncertain or when cycle timing does not follow the pattern assumed by the calculation.
- Your periods are irregular or your usual cycle length changes substantially.
- You are not sure of the first day of your last menstrual period.
- You recently stopped hormonal contraception or recently gave birth and your cycles have not become predictable.
- Pregnancy resulted from fertility treatment, where the clinic may use procedure-specific dating.
- An ultrasound or other clinical information gives your healthcare professional a different estimate.
If pregnancy dating affects a medical decision, use the date documented by your prenatal healthcare team rather than relying only on an online calculator.
Pregnancy due date calculator FAQs
Is a due date the exact day my baby will be born?
No. A due date is an estimate used to help track pregnancy. Birth may happen before or after the estimated date.
Why does pregnancy dating start from the last menstrual period?
The first day of the LMP is often easier to identify than the exact date of ovulation or conception. Gestational age is therefore commonly counted from the first day of the last menstrual cycle.
What if my menstrual cycle is not 28 days?
Select your usual cycle length in the calculator. The estimate is adjusted by the difference between your selected cycle length and 28 days. This remains an approximation because ovulation can vary from cycle to cycle.
Can an ultrasound change my estimated due date?
Yes. Prenatal ultrasound can provide additional pregnancy-dating information, and your healthcare professional may confirm or revise the estimated due date based on clinical guidance and your individual circumstances.
What if I do not know the first day of my last period?
This calculator cannot produce a dependable LMP-based estimate without that date. Speak with a qualified healthcare professional about appropriate pregnancy dating.
Does this calculator diagnose pregnancy or replace prenatal care?
No. It is an educational calculator only. It does not diagnose pregnancy, assess pregnancy health, or replace medical advice, examination, testing or prenatal care.
How this page is maintained
This page is written for general educational use. The calculator method and explanatory content are checked against established public-health and obstetric references. We aim to distinguish clearly between an estimated calculation and medical confirmation.
Last content review: October 2026
Medical-review statement: This page does not claim individual medical review by a named clinician. Medical facts are referenced to established health organizations, and users are directed to qualified healthcare professionals for personal advice.
Medical references
The following sources are provided so readers can verify the general pregnancy-dating information used on this page.