Conception Calculator
HealthChoose what you know: your last period, your due date, a conception date or an IVF transfer. The calculator estimates the conception date and the due date, with the window of days when sex could have led to the pregnancy.
Step by step
August 18, 2026 + (28 − 14) days = September 1, 2026
- Ovulation, and so conception, usually happens 14 days before the next period. With a cycle of 28 days, that is 28 − 14 days after the first day of your last period (August 18, 2026): September 1, 2026.
- Sperm survive up to 5 days and the egg 12 to 24 hours, so sex from August 27, 2026 to September 2, 2026 could have led to this conception.
- The due date is 266 days (38 weeks) after conception: May 25, 2027.
Once every entry is valid, you’ll see each step of the calculation here.
Formula
- LMP
- the first day of the last menstrual period
- cycle length
- days from the first day of one period to the first day of the next
- 266 days
- 38 weeks, the time from conception to the due date (280 days minus the 14 days before ovulation)
- embryo age
- the age of the embryo at transfer in days: 5 for a blastocyst, 3 for an earlier embryo
How it works
Doctors in the US count a pregnancy from the first day of the last menstrual period (LMP), and by convention the due date is 280 days, or 40 weeks, after it. The American College of Obstetricians and Gynecologists (ACOG) notes that this assumes a 28-day cycle with ovulation on day 14. Conception cannot happen on day 1, because an egg is released only around ovulation. So conception comes about 14 days after the LMP in a 28-day cycle and about 266 days (38 weeks) before the due date. The calculator uses that 266-day gap in both directions.
From a last period, it finds ovulation by counting back 14 days from the next period: the LMP plus the cycle length minus 14. A 32-day cycle puts conception 18 days after the LMP and the due date 4 days later than the standard one. The days when sex could have led to conception run from 5 days before to 1 day after that date. Sperm can live for 3 to 5 days inside the body and the egg lives for 12 to 24 hours after it is released, so the days just before ovulation count the most. From a due date, it subtracts 266 days, and from a known conception date it adds them. For IVF, ACOG dates the pregnancy from the embryo: the due date is 261 days after a day-5 transfer and 263 days after a day-3 transfer, so fertilization was 5 or 3 days before the transfer.
Treat the result as the middle of a window, not an exact day. Ovulation does not always fall 14 days before the next period (the Office on Women's Health says it can be 7 to 19 days before), cycles vary, and many people do not remember their last period accurately. An ultrasound in the first trimester is the most accurate way to date a pregnancy. This page does not tell you who the father is or when you will next be fertile. For the fertile days of your coming cycle, use the ovulation calculator, and to see how far along you are, use the pregnancy week calculator.
The formula
The 14 days stand for the second half of the cycle, which is fairly steady from one cycle to the next, while the first half varies. For a 28-day cycle, LMP + 14 days is the conception date and LMP + 280 days is the due date. A longer or shorter cycle moves both dates by the same number of days, the adjustment of the extended Naegele rule.
From a due date or a conception date the cycle length is unknown, so the equivalent first day of the last period is figured for a 28-day cycle. For IVF the numbers follow from 266 days after fertilization: 266 − 5 = 261 and 266 − 3 = 263 days after the transfer.
Example
Your last period began on June 1, 2026 and your cycles last 28 days. When did you most likely conceive?
- Ovulation comes 14 days before the next period, which is 28 − 14 = 14 days after the first day of the last period: June 15, 2026.
- Sex from June 10, 2026 to June 16, 2026 could have led to this conception, since sperm can wait for the egg for several days.
- Adding 266 days to the conception date gives the due date, March 8, 2027, which is the same as 280 days after the last period.
With a last period that began on June 1, 2026 and a cycle of 28 days, you most likely conceived on June 15, 2026. Sex between June 10, 2026 and June 16, 2026 could have led to it, and your due date is March 8, 2027.
Load this example into the calculatorFrequently asked questions
How accurate is conception date calculation?
It is an estimate. The standard method assumes a 28-day cycle with ovulation on day 14, and ACOG points out that it does not account for irregular cycles or variable ovulation timing. Only about half of women recall their last period accurately. Read the date as the middle of a window of several days.
What's the difference between due date and LMP calculation methods?
The LMP method counts 280 days from the first day of your last period and shifts the date with your cycle length. The due date method starts from a date your provider has already set, often adjusted by an early ultrasound, and counts back 266 days. It cannot know your cycle, so it assumes 28 days.
Why might my conception date be different than expected?
Ovulation can happen 7 to 19 days before the next period, not always 14, according to the Office on Women's Health. Sperm can survive 3 to 5 days, so sex a few days before ovulation can lead to conception. An irregular cycle or a misremembered last period also shifts the estimate.
When should I seek medical confirmation of conception timing?
Ask your provider for a dating ultrasound as early as you can, because ACOG calls first-trimester ultrasound (up to 13 weeks and 6 days) the most accurate way to date a pregnancy. Providers change the due date when the ultrasound differs from the last-period date by more than 5 days before 9 weeks of pregnancy, or by more than 7 days from 9 to 14 weeks. Go sooner if your cycles are irregular.
Can this calculator predict my most fertile days?
No. It looks back from a due date or a last period to estimate when you conceived, or forward from a conception date to the due date. To estimate the fertile window of your next cycle, use the ovulation calculator, which works from your last period and cycle length.
What should you do after you get a positive pregnancy test?
Contact your health care provider to confirm the pregnancy and to book your first prenatal visit. They will set a due date, often with an ultrasound. Meanwhile, the pregnancy week calculator shows how far along you are, and this page shows when you most likely conceived.
How do I calculate the conception date from the due date?
Subtract 266 days (38 weeks) from the due date. For a due date of March 8, 2027, conception was on June 15, 2026. Choose "Due date" in the calculator, enter the date and you also see the window of days when sex could have led to it.
How long is pregnancy from conception to birth?
About 38 weeks or 266 days. Doctors count 40 weeks from the first day of the last period, which is around 2 weeks before conception in a 28-day cycle. MedlinePlus says a normal pregnancy can last from 38 to 42 weeks of gestational age, so the due date is a midpoint, not a promise.
How does the calculator work after IVF?
Choose "IVF transfer", enter the transfer date and the age of the embryo. ACOG assigns the due date 261 days after a day-5 embryo transfer and 263 days after a day-3 transfer. Fertilization happened in the lab 5 or 3 days before the transfer, which is the conception date shown.
Can a conception date show who the father is if there are two possible fathers?
No. Sperm can survive up to 5 days and the window of conception spans about 6 days, so one pregnancy can fit more than one date. A calendar cannot settle paternity. A DNA paternity test can, and a provider can tell you how to arrange one.
Can I find out when I was conceived from my birthday?
Choose "Due date" and enter your birth date. You were conceived roughly 266 days earlier. Babies are often born a week or two before or after the due date, and normal pregnancies run 38 to 42 weeks, so the result can be off by a couple of weeks.
- Methods for Estimating the Due Date (Committee Opinion No. 700) (American College of Obstetricians and Gynecologists)
- Your Menstrual Cycle (Office on Women’s Health, U.S. Department of Health and Human Services)
- Gestational age (MedlinePlus, National Library of Medicine)
- Pregnancy Test (MedlinePlus, National Library of Medicine)
- Naegele’s rule (Wikipedia)