Estimate conception date, due date, gestational age, fetal age, and key pregnancy timing from your LMP, conception date, due date, or IVF transfer date.
Advanced options
How to use our Pregnancy Conception Calculator
- Choose What date do you know?, then enter the matching Known date.
- If you chose LMP, enter your usual Cycle length (days); if you chose IVF, choose Embryo age at transfer (days).
- Set Calculate age as of (date) to today or any date you want to check.
- Click Calculate to see Estimated conception date, Estimated due date, Gestational age on as-of date, and Estimated fetal age on as-of date.
- Sanity-check the result by comparing Gestational age on as-of date with Estimated fetal age on as-of date; in most non-IVF cases, fetal age is about 2 weeks less.
Definitions
Known date: The one date you already know and want to start from, such as LMP, conception, due date, or IVF transfer date.
Cycle length (days): The usual number of days from the first day of one period to the first day of the next period.
Embryo age at transfer (days): How many days old the embryo was when it was transferred in IVF, usually 3 or 5.
Estimated conception date: The best calculated date for fertilization. It is exact only when conception timing is truly known, such as some IVF cases.
Estimated due date: The estimated day the pregnancy reaches 40 weeks by standard obstetric dating.
Gestational age on as-of date: Pregnancy age counted from the first day of the last menstrual period or an LMP-equivalent date, which is the standard medical timeline [3].
Estimated fetal age on as-of date: Time since estimated conception. This is usually about 2 weeks less than gestational age in non-IVF dating.
LMP-equivalent start date: The date this calculator uses as pregnancy day 1 so all methods can be shown on the same gestational timeline.
Trimester on as-of date: A simple stage label based on gestational age: first, second, or third trimester.
Days until due date: The number of days from the as-of date to the estimated due date. A negative number means the as-of date is after the due date.
Common mistakes and quick fixes
Mistake: Entering a future Known date in LMP, Conception date, or IVF transfer date mode.
Fix: Use the actual past date, or switch What date do you know? to Due date if the only date you know is a future due date.
Mistake: Leaving Cycle length (days) at 28 when your usual cycle is much longer or shorter.
Fix: In LMP mode, enter your usual cycle length so Estimated conception date and Estimated fetal age on as-of date shift correctly.
Mistake: Using the date bleeding started late in a cycle instead of the first day of the last true period for Known date .
Fix: In LMP mode, enter the first day of your last menstrual period so Gestational age on as-of date and Estimated due date use standard pregnancy dating.
Mistake: Picking the wrong Embryo age at transfer (days) in IVF mode.
Fix: Match the transfer record exactly, such as Day 3 embryo or Day 5 embryo, because it changes the LMP-equivalent start date and Estimated due date .
Mistake: Thinking Estimated fetal age on as-of date should always equal Gestational age on as-of date .
Fix: Read them differently: gestational age is counted from the LMP-equivalent start date , while fetal age is counted from Estimated conception date .
Mistake: Assuming a negative Days until due date means the result is broken.
Fix: A negative value simply means your Calculate age as of (date) is after the Estimated due date .
Limitations & Key Assumptions / Boundary Conditions
- This calculator uses standard obstetric dating rules: 280 days from LMP and 266 days from conception.
- In LMP mode, ovulation is estimated as Cycle length (days) - 14, so irregular cycles can make Estimated conception date less precise.
- Estimated due date is still an estimate. Birth does not always happen on that exact date.
- If your period dates are uncertain, your cycles are irregular, or symptoms and dates do not match, a clinician may confirm or revise dating with ultrasound.
- The calculator blocks results if Calculate age as of (date) is before the pregnancy start used for gestational dating.
- In IVF mode, results depend on choosing the correct Embryo age at transfer (days).
- This tool does not replace clinical dating decisions, especially when ultrasound findings differ from calendar-based estimates.
Methodology
How the calculator dates pregnancy
This tool first converts your chosen starting point into two timeline anchors: an Estimated conception date and an LMP-equivalent start date. It then uses those dates to show due date, gestational age, fetal age, trimester, and days until due.
Core formulas
Estimated due date = LMP + 280 days
Estimated conception date = LMP + (Cycle length - 14) days
Estimated due date = Conception date + 266 days
Estimated conception date = Due date - 266 days
Estimated due date = IVF transfer date + (266 - embryo age) days
LMP-equivalent start date = Conception date - 14 days
LMP-equivalent start date = IVF transfer date - embryo age - 14 days
Gestational age in days = As-of date - LMP-equivalent start date
Estimated fetal age in days = As-of date - Estimated conception date
Days until due date = Estimated due date - As-of date
How each mode works
LMP mode: The due date comes from 280 days after the first day of the last menstrual period. Conception is estimated from cycle length, but gestational age still starts from LMP.
Conception date mode: The due date is 266 days after conception, and the calculator sets the LMP-equivalent start 14 days earlier so gestational age matches standard medical counting [3].
Due date mode: The calculator works backward 266 days to estimate conception, then places the LMP-equivalent start 14 days before that.
IVF transfer mode: The tool uses the transfer date and embryo age to reconstruct conception timing and the LMP-equivalent start. This is useful because IVF timing is usually known more precisely than natural-cycle conception timing [3].
Worked mini-example
Example: if What date do you know? is LMP, Known date is January 1, 2026, Cycle length (days) is 28, and Calculate age as of (date) is March 18, 2026:
Estimated conception date = January 1, 2026 + (28 - 14) days = January 15, 2026
Estimated due date = January 1, 2026 + 280 days = October 8, 2026
Gestational age = March 18, 2026 - January 1, 2026 = 76 days = 10 weeks 6 days
Estimated fetal age = March 18, 2026 - January 15, 2026 = 62 days = 8 weeks 6 days
Days until due date = October 8, 2026 - March 18, 2026 = 204 days
That is why gestational age and fetal age can both be correct while showing different numbers.
Interpreting the outputs
Gestational age on as-of date is the standard week count most clinicians use. Estimated fetal age on as-of date is closer to time since conception, so it is usually about 2 weeks lower in non-IVF pregnancies. If the as-of date falls before estimated conception, the tool shows that conception had not happened yet on that date instead of showing a negative fetal age.
Assumptions behind the math
Standard pregnancy dating commonly uses 280 days from LMP and 266 days from conception [1]. Early ultrasound can be used clinically to confirm or revise the estimated due date, especially when period dates are uncertain or cycles are irregular [1].