Your due date anchors the whole pregnancy timeline: it sets when screening tests are offered, how growth is judged and when a baby counts as full term. This calculator estimates it from the information you have — the first day of your last period, a known conception date or an IVF embryo transfer — and then lays out how far along you are, your trimester dates and a week-by-week timeline.
How to use the pregnancy calculator
- Choose what to calculate from: last menstrual period (LMP), conception date or IVF transfer.
- Enter the matching date. With LMP, also enter your usual cycle length; with IVF, choose a 3-day or 5-day embryo.
- The Show progress as of date defaults to today. Change it to see how far along you will be on any future date, such as a scheduled appointment.
- Read the due date and gestational age on the tape, then scroll for the trimester and week-by-week tables.
How due dates are calculated
Each method converts your date into a due date (EDD, estimated date of delivery):
The LMP formula is Naegele’s rule, named for the German obstetrician Franz Naegele, who popularized it in the early 1800s. Weeks of pregnancy are then counted from a “dating LMP” set exactly 280 days before the due date, so every method produces the same week numbers for the same due date.
Worked example
Last period began January 1, 2026, with a 28-day cycle: January 1 + 280 days = Thursday, October 8, 2026.
The same answer by hand: add one year (January 1, 2027), subtract three months (October 1, 2026), add seven days (October 8, 2026).
On March 12, 2026 the pregnancy is exactly 70 days along — 10 weeks, 0 days — with 210 days to go.
With a 32-day cycle, the due date shifts four days later, to October 12, 2026. A 5-day blastocyst transferred on January 20, 2026 gives January 20 + 261 days = October 8, 2026.
Trimesters and term
The American College of Obstetricians and Gynecologists (ACOG) divides pregnancy by completed weeks of gestation:
| Stage | Gestational age |
|---|---|
| First trimester | 0w 0d – 13w 6d |
| Second trimester | 14w 0d – 27w 6d |
| Third trimester | 28w 0d – birth |
| Early term | 37w 0d – 38w 6d |
| Full term | 39w 0d – 40w 6d |
| Late term | 41w 0d – 41w 6d |
| Postterm | 42w 0d and later |
“10w 3d” means 10 completed weeks plus 3 days — you are in the eleventh week of pregnancy.
How reliable is a due date?
A due date is an estimate of the midpoint of a range. Only about one baby in twenty arrives on the exact day, and births between 37 and 42 weeks are all considered term. Period-based dates are least precise when cycles are irregular, when the LMP is uncertain, or after recently stopping hormonal contraception. A first-trimester ultrasound measuring crown-rump length is accurate to within about five to seven days, so clinicians often adjust the date after that scan. IVF dates are the most exact because the embryo’s age is known.
About the week-by-week table
The timeline lists the dates of every week from the dating LMP to 42 weeks, along with common developmental milestones and the usual windows for prenatal tests such as cell-free DNA screening, the anatomy scan, glucose screening and group B strep testing. Your own care schedule may differ. For a closer look at dating by ultrasound, see the gestational age calculator, and to plan healthy weight gain, try the pregnancy weight gain calculator.
This calculator is informational only and is not medical advice. Your healthcare professional's dating, usually confirmed by ultrasound, should guide your care.
Frequently asked questions
How is my due date calculated?
The standard method, Naegele's rule, adds 280 days (40 weeks) to the first day of your last menstrual period. A quick hand version is to add one year, subtract three months and add seven days. If you know the conception date instead, add 266 days.
Why does my cycle length change the due date?
Naegele's rule assumes ovulation on day 14 of a 28-day cycle. With a longer cycle you probably ovulated later, so the due date moves later by the same number of days; with a shorter cycle it moves earlier. A 32-day cycle adds four days.
Why is pregnancy counted from the last period instead of conception?
The first day of the last period is a date most people know, while the exact day of ovulation usually is not. Counting from the period adds about two weeks before conception, which is why a pregnancy is described as 40 weeks long even though the baby develops for about 38.
How is an IVF due date calculated?
Because the embryo's age is known exactly, the due date is the transfer date plus 266 days minus the embryo's age: 261 days after a 5-day blastocyst transfer and 263 days after a 3-day transfer. This makes IVF due dates the most precise of any method.
Can my due date change after an ultrasound?
Yes. A first-trimester ultrasound that measures crown-rump length is the most accurate way to date a pregnancy, and ACOG guidance calls for changing the due date when the scan differs from the period-based date by more than a set number of days. Your clinician's date should take priority over any calculator.