Work out your estimated conception date from your last menstrual period or due date. Understand how conception timing affects your EDD.
A pregnancy conception calculator estimates when you conceived and maps out your pregnancy timeline. It works from your last period, due date, ultrasound or IVF transfer date. Because ovulation timing varies, the conception date is an estimated window of a few days. IVF pregnancies have the most precise dating, since the transfer date is known exactly.
| Event | Date |
|---|
The calculator works from whichever date you know, then estimates conception and maps your pregnancy weeks.
These two counts differ by about two weeks, which often confuses new parents. Doctors use gestational age.
| Measure | Counted From | At Conception |
|---|---|---|
| Gestational age | First day of last period | Already ~2 weeks |
| Conception (fetal) age | Day of fertilisation | 0 weeks |
This is why you can be "6 weeks pregnant" only about 4 weeks after conception.
IVF pregnancies have the most accurate dating because the timing is known precisely. The reference date is the embryo transfer, adjusted for embryo age.
| IVF Method | Reference Date | Conception Equivalent |
|---|---|---|
| Day-3 embryo transfer | Transfer date | Conception ≈ 3 days before transfer |
| Day-5 blastocyst transfer | Transfer date | Conception ≈ 5 days before transfer |
| Frozen transfer (FET) | Transfer date | Same as fresh, by embryo age |
| Donor egg/embryo | Transfer date | Dated from transfer, not your cycle |
Once conception is estimated, the early pregnancy journey follows a predictable sequence.
| Event | Approx. Timing |
|---|---|
| Ovulation & fertilisation | Around day 14 of the cycle |
| Implantation | ~6–10 days after fertilisation |
| Positive pregnancy test | Around the missed period (~4 weeks) |
| First ultrasound | Often ~7–8 weeks |
| End of first trimester | 12 weeks |
| Estimated due date | 40 weeks from last period |
Conception occurs when a sperm fertilises an egg, typically within 24 hours of ovulation. Ovulation usually happens 14 days before the next expected period (not 14 days after LMP, unless cycles are exactly 28 days).
Sperm can survive 3–5 days in the fallopian tubes, so the fertile window extends from approximately 5 days before ovulation to the day of ovulation itself. The chance of conception is highest on the day of ovulation and the day before.
Pregnancy dating can be approached from multiple reference points depending on what information is available. This calculator handles all three common scenarios: dating from the last menstrual period (most common clinical approach), from a known conception date (useful if tracking ovulation), or backwards from the due date to determine conception and LMP dates.
| Reference Point | Relationship to LMP | Notes |
|---|---|---|
| Last Menstrual Period (LMP) | Day 0 (gestational counting starts here) | First day of last period before pregnancy |
| Conception/fertilisation | LMP + ~14 days (28-day cycle) | May be 1-5 days before or after ovulation day |
| Positive pregnancy test | LMP + ~28-35 days | Approx 2 weeks after missed period |
| First heartbeat (transvaginal US) | LMP + ~42 days (6 weeks) | Approximately 6 weeks gestational age |
| Estimated due date (EDD) | LMP + 280 days | Naegele's rule; only ~5% born on exact date |
Why do doctors count pregnancy from the last period?
Doctors count pregnancy from the first day of the last menstrual period (LMP) because this date is usually known precisely, while the exact conception date is often uncertain. Conception typically occurs approximately 14 days after LMP (for a 28-day cycle), so using LMP adds approximately 2 weeks to the gestational age compared to counting from conception. This is why pregnancy is said to last '40 weeks from LMP' when biological gestation from fertilisation is approximately 38 weeks.
What is the difference between gestational age and fetal age?
Gestational age counts from LMP — so a baby measured at 8 weeks gestational age is approximately 6 weeks old from fertilisation. Fetal age (also called conceptional age or embryonic age) counts from fertilisation. Clinical practice uses gestational age; the calculator can display both. Ultrasound measurements use gestational age for all standard reporting.
How do I calculate my conception date in pregnancy?
Work from a date you know: add about 14 days to your last period, or subtract 266 days from your due date. For IVF, dating is based on your transfer date and embryo age. Enter your details in the calculator to see your estimated conception date and pregnancy timeline.
How accurate is a pregnancy conception calculator?
It gives a good estimate but not an exact day. Natural conception depends on ovulation timing, which varies between cycles, so the result is a window of a few days. IVF pregnancies are dated very precisely. Ultrasound dating is the most accurate for naturally conceived pregnancies.
What is the difference between conception age and gestational age?
Gestational age is counted from the first day of your last period, while conception (fetal) age is counted from fertilisation. Gestational age is about two weeks ahead. Doctors use gestational age, which is why you are considered around two weeks pregnant at the moment of conception.
How does IVF change conception dating?
IVF dating is more accurate because the timing is known. Conception is dated from your embryo transfer, adjusted for the embryo’s age — for example, about five days before a day-5 blastocyst transfer. Frozen and donor transfers use the same approach, based on transfer date rather than your menstrual cycle.
How many weeks pregnant am I?
Pregnancy weeks are counted from the first day of your last period, so you are about two weeks "pregnant" at conception. Enter your last period date or conception date in the calculator to see your current week and trimester. An early ultrasound can confirm or adjust this.
Can conception happen days after intercourse?
Yes. Sperm can survive in the body for up to about five days, so fertilisation may occur several days after sex, whenever the egg is released. This is why the intercourse date and the actual conception date can differ, and why conception is estimated as a short window.
When should I see a healthcare provider?
If you have a positive pregnancy test, book with your doctor or midwife to confirm the pregnancy and arrange early antenatal care and dating. Seek prompt advice for severe pain, heavy bleeding or other concerning symptoms. A provider can give accurate dating and personalised guidance that a calculator cannot.
Can ultrasound determine my conception date?
An early ultrasound gives the most accurate pregnancy dating for naturally conceived pregnancies by measuring the baby directly. It estimates conception within a few days, especially in the first trimester. If the ultrasound dates differ from your last-period estimate, providers usually use the ultrasound date.
Can I calculate conception with irregular periods?
Yes, but last-period estimates are less reliable if your cycle is irregular, because ovulation timing is harder to predict. In that case, an early ultrasound gives more accurate dating. If you tracked ovulation or used fertility treatment, those dates improve the estimate.