How to use this calculator
Pick the method that matches what you know:
- First day of last period (LMP): enter the date your last period started, and your usual cycle length if it isn’t 28 days.
- Conception date: if you know when you conceived, for example from ovulation tracking.
- IVF embryo transfer date: enter the transfer date and whether the embryo was a day-3 or day-5 (blastocyst) embryo.
- Ultrasound dating: enter the date of a dating scan and the gestational age it gave, in weeks and days.
The show progress as of date defaults to today; change it to see how far along you’ll be on any date. You’ll get:
- your estimated due date
- your gestational age in weeks and days, your trimester and where you are on the 40-week scale
- your likely conception window
- a milestones table with dates for each trimester, the anatomy scan window and the ACOG term categories
How due dates are calculated
Every method converts what you know into the same reference point: the first day of a 280-day (40-week) pregnancy, counted from the last menstrual period.
From your last period. This is Naegele’s rule: add 280 days to the first day of your LMP, which is the same as adding one year, subtracting three months and adding seven days.
The cycle adjustment assumes you ovulate about 14 days before your next period, so a 32-day cycle moves the due date four days later.
From conception. Conception usually happens around two weeks after the LMP, so the due date is 266 days (38 weeks) after conception.
From an IVF transfer. Following ACOG, the due date is 261 days after a day-5 embryo transfer and 263 days after a day-3 transfer. The embryo’s age is already known, so these dates are precise.
From an ultrasound. The due date is the scan date plus the days remaining to 40 weeks: 280 minus the gestational age at the scan, in days.
Gestational age on any date is that date minus the start of the pregnancy (the LMP-equivalent date), shown in completed weeks and days.
Worked example
Your last period started on September 1, 2026, and your cycles are 28 days.
- September 1, 2026 + 280 days = June 8, 2027, your estimated due date.
- On November 6, 2026, you’re 66 days along: 9 weeks 3 days, in the first trimester.
- Ovulation was likely around September 15, so the likely conception window is September 10 to 15: the six-day fertile window ending on ovulation day.
The other methods agree when the inputs line up: conception on September 15, a day-5 transfer on September 20, a day-3 transfer on September 18, or an ultrasound on October 20 measuring 7 weeks 0 days all give a due date of June 8, 2027. With a 32-day cycle instead, the LMP method gives June 12.
Pregnancy milestones
For the example above, here’s when each milestone falls:
| Milestone | Gestational age | Date |
|---|---|---|
| End of first trimester | 13w 6d | Dec 7, 2026 |
| Anatomy ultrasound window | 18w 0d – 22w 6d | Jan 5 – Feb 8, 2027 |
| Periviable period | 20w 0d – 25w 6d | Jan 19 – Mar 1, 2027 |
| Third trimester begins | 28w 0d | Mar 16, 2027 |
| Early term begins | 37w 0d | May 18, 2027 |
| Full term begins | 39w 0d | Jun 1, 2027 |
| Estimated due date | 40w 0d | Jun 8, 2027 |
| Late term begins | 41w 0d | Jun 15, 2027 |
| Postterm begins | 42w 0d | Jun 22, 2027 |
A few notes on these milestones:
- Anatomy ultrasound. ACOG notes that a standard ultrasound exam is usually done between 18 and 22 weeks to check the fetus’s anatomy and growth.
- Periviable period. ACOG and the Society for Maternal-Fetal Medicine define periviable birth as delivery from 20 weeks 0 days through 25 weeks 6 days. Outcomes for babies born in this window vary widely from week to week.
- Term categories. In 2013 ACOG replaced the single “term” label with early term (37w 0d to 38w 6d), full term (39w 0d to 40w 6d), late term (41w 0d to 41w 6d) and postterm (42w 0d and later), because outcomes for babies differ across that five-week span.
What your due date means
Your due date marks 40 weeks, the midpoint of the full-term window, rather than a deadline. Labor can start naturally on any day in the term range, so it helps to think of a due date as the center of a window rather than an appointment.
ACOG recommends settling the due date as soon as the LMP, a first accurate ultrasound, or both are available, and changing it afterward only in rare cases. A first-trimester ultrasound, up to 13 weeks 6 days, is the most accurate method. If an early scan disagrees with your period dates by more than a set amount, which depends on how far along you are, your clinician will usually go with the ultrasound date. After an IVF transfer, the transfer-based date is used.
Assumptions and limitations
- The LMP method assumes regular cycles with ovulation about 14 days before the next period. Irregular cycles, recent hormonal contraception or breastfeeding make it less reliable.
- The conception window is an estimate based on typical ovulation timing, and the fertile window research behind it comes from couples trying to conceive naturally.
- Ultrasound dating is less precise later in pregnancy; this calculator takes the scan’s gestational age as given.
- Trimester boundaries vary slightly between sources. This calculator uses 13w 6d and 27w 6d as the ends of the first and second trimesters.
Sources
- American College of Obstetricians and Gynecologists. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol. 2017;129(5):e150–e154.
- American College of Obstetricians and Gynecologists. Committee Opinion No. 579: Definition of Term Pregnancy. Obstet Gynecol. 2013;122(5):1139–1140.
- American College of Obstetricians and Gynecologists and Society for Maternal-Fetal Medicine. Obstetric Care Consensus No. 6: Periviable Birth. Obstet Gynecol. 2017;130(4):e187–e199.
- American College of Obstetricians and Gynecologists. Ultrasound Exams (patient FAQ).
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med. 1995;333(23):1517–1521.
This calculator gives an estimate for general information only. Your clinician will confirm your due date, usually with an early ultrasound.
Frequently asked questions
How is a due date calculated?
The standard method adds 280 days (40 weeks) to the first day of your last menstrual period. For a last period starting September 1, 2026, that gives June 8, 2027. If your cycles are longer or shorter than 28 days, this calculator shifts the date by the difference.
How accurate is my due date?
It's an estimate of when you'll reach 40 weeks, not a prediction of the day labor starts. According to ACOG, an ultrasound in the first trimester, up to 13 weeks 6 days, is the most accurate way to establish or confirm the due date, and your clinician may adjust it after one.
Why is pregnancy counted from my last period and not conception?
The first day of the last period is a date most people know, while conception usually isn't. By convention, gestational age starts there, about two weeks before conception in a 28-day cycle. That's why you're considered "4 weeks pregnant" only about two weeks after conceiving.
How do you calculate an IVF due date?
ACOG advises dating IVF pregnancies from the embryo's age and the transfer date. For a day-5 embryo (blastocyst), the due date is 261 days after transfer; for a day-3 embryo, it's 263 days after transfer.
What do early term, full term and late term mean?
ACOG and the Society for Maternal-Fetal Medicine define early term as 37 weeks 0 days through 38 weeks 6 days, full term as 39 weeks 0 days through 40 weeks 6 days, late term as 41 weeks 0 days through 41 weeks 6 days, and postterm as 42 weeks 0 days or later. Babies born before 37 weeks are preterm.
When does each trimester start?
This calculator uses the common clinical boundaries. The first trimester runs through 13 weeks 6 days, the second from 14 weeks 0 days through 27 weeks 6 days, and the third from 28 weeks 0 days until birth.
Updated November 2026. Results are estimates for educational purposes and depend on the inputs you provide.