Estimate when conception happened from your due date or last period
Updated October 2026
Due date is the date on your prenatal record. If an early ultrasound changed it, use the updated one, because it is the better estimate. First day of your last period means the first day of real bleeding, not spotting and not the last day. Cycle length counts from the first day of one period to the first day of the next; the calculator accepts 21 to 45 days and only uses it when you start from your last period.
The result shows three dates. The estimated conception (ovulation) date is the center of the likely window. The due date is the matching 40-week date. The LMP line is the period start date that dating counts from; it is marked "estimated" when you started from a due date.
Doctors date pregnancy from the last menstrual period (LMP): by convention the due date is 280 days after the LMP. Ovulation in a textbook 28-day cycle falls on day 14, so conception is taken as 266 days before the due date.
Starting from a due date of July 15, 2027: July 15, 2027 minus 266 days is October 22, 2026 (the conception estimate), and minus 280 days is October 8, 2026 (the LMP). Starting instead from an October 8, 2026 period with a 32-day cycle: ovulation moves to day 18 (32 − 14), giving conception about October 26, 2026 and a due date of July 19, 2027, four days later than in a 28-day cycle.
Same first day of the last period (October 8, 2026), different cycle lengths:
| Cycle length | Likely ovulation | Estimated conception | Due date |
|---|---|---|---|
| 24 days | Day 10 | October 18, 2026 | July 11, 2027 |
| 28 days | Day 14 | October 22, 2026 | July 15, 2027 |
| 32 days | Day 18 | October 26, 2026 | July 19, 2027 |
| 35 days | Day 21 | October 29, 2026 | July 22, 2027 |
The calculator assumes the second half of the cycle (ovulation to the next period) is 14 days. That is the usual textbook figure, and it is why longer cycles shift ovulation later.
The American College of Obstetricians and Gynecologists (ACOG Committee Opinion 700, 2017) sets the rules U.S. clinicians use. The due date is first set from the LMP, then confirmed or changed by the earliest ultrasound. A first-trimester crown-rump length measurement is accurate to about ±5 to 7 days. ACOG notes that only about half of women recall their LMP accurately. The due date is changed when the ultrasound and LMP dates differ by more than these amounts:
| Gestational age at ultrasound (weeks) | Measurement | Redate if LMP and ultrasound differ by |
|---|---|---|
| Up to 8 6/7 | Crown-rump length | More than 5 days |
| 9 0/7 to 13 6/7 | Crown-rump length | More than 7 days |
| 14 0/7 to 15 6/7 | Biometry | More than 7 days |
| 16 0/7 to 21 6/7 | Biometry | More than 10 days |
| 22 0/7 to 27 6/7 | Biometry | More than 14 days |
| 28 0/7 and later | Biometry | More than 21 days |
Once set, the due date should rarely change again. For IVF pregnancies the embryo’s age and transfer date set the due date instead (see the IVF due date calculator).
In a study of 221 women trying to conceive, every pregnancy came from intercourse during a six-day window ending on the day of ovulation (Wilcox et al., 1995). For the default due date, that window runs from about October 17, 2026 to October 22, 2026. Even when ovulation is measured exactly, pregnancy length varies: in 125 naturally conceived births, the median from ovulation to birth was 268 days, and the spread was 37 days even after excluding preterm births (Jukic et al., 2013). A due date alone therefore cannot point to one day of conception.
Treating conception as the day of intercourse. Sperm can survive several days, so intercourse up to five days before ovulation can lead to pregnancy. Entering the last day of the period instead of the first moves every date about five days late. Using a 28-day default with long or irregular cycles puts ovulation too early; with irregular cycles, an ultrasound-based due date is the better starting point. Mixing up gestational age and fetal age: a pregnancy called "6 weeks" is about 4 weeks after conception. Using this page for IVF: with assisted reproduction, the exact dates are known, so use the transfer date.
This page gives an estimate, not a medical or legal finding. Your clinician sets the official due date, usually from an early ultrasound, so book a prenatal visit once a home test is positive. If the timing matters for paternity, the date of conception cannot settle it, because the window spans days and pregnancy length varies by weeks; only DNA testing can. Call your clinician or seek urgent care for any of the CDC’s urgent maternal warning signs, including vaginal bleeding or fluid leaking, severe belly pain that doesn’t go away, a severe or worsening headache, vision changes, fever of 100.4°F or higher, trouble breathing, chest pain, or fainting.