How to Use the Due Date Calculator
Pick the dating method that matches what you know, then read the estimated due date and related dates.
- Choose a mode: Last period (LMP), Conception date, IVF transfer, or Ultrasound.
- Enter the date for that mode. For LMP, set average cycle length. For IVF, choose Day 3, Day 5, or Day 6. For ultrasound, enter gestational weeks and days on the scan date.
- Results update live — no submit button. Review the estimated due date, weeks pregnant (when the pregnancy is ongoing), trimester, and key dates table.
- Use Reset to clear inputs. Dates never appear in the URL and are not stored.
How Your Due Date Is Calculated
Four common dating methods, each producing an estimated due date (EDD).
Last period (LMP): EDD = first day of the last menstrual period + 280 days + (cycle length − 28). Pregnancy weeks are counted from that LMP date, so conception is usually around week 2 on this calendar.
Conception date: EDD = conception date + 266 days.
IVF transfer: EDD = transfer date + 266 days − embryo age in days (Day 3, Day 5, or Day 6).
Ultrasound: EDD = scan date + (280 − gestational age in days on that date).
Worked example (illustrative calendar dates, no specific year): last period on March 15 with a 28-day cycle → estimated due date December 20. That is March 15 + 280 days.
How Accurate Is an Estimated Due Date?
An honest look at what an EDD can and cannot tell you.
An estimated due date is a midpoint used for planning and gestational dating. Only a small percentage of births happen on that exact calendar day. Term pregnancy is a range — most births occur between 37 and 42 weeks.
Period dating assumes a typical cycle and ovulation timing. Early ultrasound can be more accurate than LMP dating alone, especially with irregular cycles. Healthcare providers may adjust the date when clinical information supports a change — this tool does not replace that judgment.
Pregnancy Trimesters and Term Definitions
Simple ranges used on this page for a trimester calculator view.
| Category | Range |
|---|---|
| First trimester | 0w0d to 13w6d |
| Second trimester | 14w0d to 27w6d |
| Third trimester | 28w0d onward |
| Early term | 37w0d to 38w6d |
| Full term | 39w0d to 40w6d |
| Late term | 41w0d to 41w6d |
| Postterm | 42w0d and beyond |
Exact trimester and term definitions vary slightly by source. Your healthcare provider’s definitions and charting take priority.
Frequently Asked Questions
LMP dating, conception and IVF due dates, ultrasound changes, and how pregnancy weeks are counted.
How is my due date calculated?
This due date calculator estimates an EDD from one of four inputs: last menstrual period (LMP) with cycle length, conception date, IVF embryo transfer (with embryo age), or an ultrasound with gestational age on the scan date. All modes compute an estimated due date, then derive weeks pregnant and key dates from that EDD.
How accurate is an estimated due date?
An estimated due date is a planning estimate, not a birth prediction. Only a small percentage of births happen on the exact date. Most births occur between 37 and 42 weeks. An early ultrasound can be more accurate than period dating alone, especially with irregular cycles, and a healthcare provider may adjust the date.
What if my cycle is not 28 days long?
In last-period mode, enter your average cycle length. The calculator adjusts from a 28-day baseline: EDD = LMP + 280 days + (cycle length − 28). Cycles outside the supported range are better discussed with a healthcare professional, and ultrasound dating may help.
How do I calculate my due date from my conception date?
Choose the Conception date mode and enter the date. The estimated due date is the conception date plus 266 days. Pregnancy weeks are still counted from a theoretical last period about two weeks earlier, which is why gestational age can look ahead of the conception date.
How is the due date calculated with IVF?
In IVF transfer mode, enter the transfer date and whether the embryo was Day 3, Day 5, or Day 6. The estimated due date is the transfer date plus 266 days, minus the embryo age in days. Your clinic’s dating may still differ slightly — follow their guidance.
Can an ultrasound change my due date?
Yes. When a scan reports gestational age on a given date, this calculator estimates the due date as scan date plus the remaining days to 40 weeks. Healthcare providers often prefer early ultrasound dating when it differs meaningfully from period dating, especially if cycles are irregular.
How are weeks of pregnancy counted?
Pregnancy weeks are usually counted from the first day of the last menstrual period, not from conception. Conception typically occurs around week 2 on that calendar. That is why a pregnancy can be dated as several weeks along even shortly after conception.
What if I am not sure of my last period date?
If the last period date is uncertain, try the Ultrasound mode with a dating scan’s gestational age, or ask a midwife, doctor, or other healthcare professional. They can confirm dating and a care plan.
Sources
Methods for estimating the due date: ACOG Committee Opinion — Methods for Estimating the Due Date. General information on estimated due dates and the 37–42 week window: NHS — Pregnancy due date calculator.
This calculator provides general information and is not medical advice. Your healthcare provider will confirm your due date and care plan.
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