Conception Date Calculator
Estimate when conception occurred using your LMP, due date, ultrasound measurements, birth date, or IVF transfer information
Choose the method that matches the information you have available
Enter the first day of your last menstrual period before pregnancy was detected
Your typical menstrual cycle length in days (range: 20–45 days). Default is 28 days.
Enter Your Information to Calculate
Select a calculation method and enter the required date above. Results including conception date, fertile window, trimester timeline, and clinical milestones will appear here.
How to Use the Conception Date Calculator
Choose Your Calculation Method
Select the tab that matches the information you have available. Use 'From LMP' if you know the first day of your last period before pregnancy — this is the most common starting point. Use 'From Due Date' if you know your estimated delivery date from an early appointment. Use 'From Ultrasound' if you have gestational age measurements from a scan. Use 'From Birth Date' to reverse-calculate conception from a baby's birthday. Use 'IVF / ART' if your pregnancy was conceived through assisted reproductive technology.
Enter Your Dates and Cycle Details
For the LMP method, enter your LMP date and your average cycle length. If your cycles are irregular or you track your luteal phase, expand the Advanced Options section to enter a custom luteal phase length. For the ultrasound method, enter both the scan date and the gestational age in weeks and days exactly as shown on your ultrasound report. For IVF, select your transfer type carefully — Day 3 and Day 5 transfers produce different due date calculations.
Review Your Conception Date and Key Dates
After entering your information, results appear instantly. The top section shows your estimated conception date prominently. Below it, review the conception window, ovulation date, fertile window, and estimated due date. The current gestational age is calculated from today's date automatically. The pregnancy timeline bar shows your position in the pregnancy with all three trimester boundaries. Scroll down to see clinical milestone dates and the implantation probability chart.
Use and Share Your Results
Use the Copy Results button to copy a plain-text summary to your clipboard for sharing with a partner or saving in notes. Use the Export button to download a text file of all results. Use the Print button to print results for a medical appointment or personal records. Note that all dates shown are estimates; your healthcare provider may adjust the official due date based on early ultrasound measurements, which is standard practice and does not indicate a problem.
Frequently Asked Questions
How accurate is the conception date calculator?
The accuracy of the conception date estimate depends on which calculation method you use and the accuracy of your inputs. The LMP method (from last menstrual period) is accurate to within about 5 to 7 days for women with regular 28-day cycles, but becomes less accurate as cycle length varies from the norm. First-trimester ultrasound is the gold standard for pregnancy dating, accurate to within 5 to 7 days regardless of cycle regularity. The due date reverse method has the same accuracy as its source — if your due date was established by early ultrasound, the resulting conception date is equally reliable. IVF dating is the most precise of all methods, since the egg retrieval date is known exactly. Birth date reverse calculation has the widest margin of error — roughly plus or minus two weeks — because birth timing naturally varies from 38 to 42 weeks. For all methods, remember that actual conception requires both ovulation and viable sperm meeting at the right time, which introduces inherent biological uncertainty regardless of calculation precision.
What is the difference between gestational age and fetal age?
Gestational age and fetal age differ by approximately two weeks, which causes frequent confusion. Gestational age counts from the first day of the last menstrual period (LMP), even though conception did not occur until ovulation approximately two weeks later. This convention was established before ultrasound existed, when the LMP was the only known date in early pregnancy. The fetal age (also called conceptional age or fertilization age) counts from the actual conception date. When a healthcare provider says you are 10 weeks pregnant, they mean 10 weeks gestational age — which corresponds to approximately 8 weeks since actual fertilization occurred. A full-term pregnancy is 40 weeks gestational age, which equals 38 weeks fetal age. This calculator shows gestational age to match clinical convention, but also shows your conception date so you can understand both timelines.
Why does my cycle length matter for conception date calculation?
Cycle length directly determines when ovulation occurs, and ovulation is the key event that conception depends on. In a standard 28-day cycle, ovulation occurs around day 14. In a 35-day cycle, ovulation occurs around day 21. In a 24-day cycle, ovulation occurs around day 10. If your cycle is 35 days and a calculator assumes 28 days, it will estimate your ovulation — and therefore conception — about a week earlier than it actually happened. This seven-day error translates directly into a seven-day error in the conception date estimate. For women with irregular cycles, the LMP method becomes significantly less reliable, which is why early ultrasound is so valuable. Entering your actual average cycle length in this calculator improves the accuracy of the LMP-based estimate substantially compared to tools that assume 28 days.
What is the fertile window and how long is it?
The fertile window is the period during each menstrual cycle when sexual intercourse can potentially result in conception. It spans the five days before ovulation plus ovulation day itself — a total of six days. The window is defined by two biological facts: sperm can survive inside the female reproductive tract for up to five days (occasionally as long as seven days), and the egg is viable for only 12 to 24 hours after ovulation. This means intercourse on any of the five days before ovulation can still result in fertilization because sperm remain viable until ovulation occurs. The highest pregnancy probability comes from intercourse one to two days before ovulation. Intercourse on the day of ovulation or after carries lower probability because the egg's viability window is short. This calculator shows the full six-day fertile window and marks the most likely intercourse dates that could have led to a specific pregnancy.
How does the IVF conception date calculation work?
For IVF pregnancies, the biological conception equivalent is the day of egg retrieval, when fertilization takes place in the laboratory. The transfer date — when the embryo is placed in the uterus — occurs 3 days after retrieval for cleavage-stage embryos (Day 3 transfer) or 5 days after retrieval for blastocyst-stage embryos (Day 5 transfer). For gestational age counting, IVF providers use the LMP convention: for a Day 3 transfer, the estimated LMP is 16 days before the transfer date; for a Day 5 transfer, the estimated LMP is 18 days before the transfer date. This means an IVF patient is already counted as approximately 2 weeks pregnant at egg retrieval, following the same convention as natural pregnancy. The due date is then calculated as the LMP equivalent plus 280 days. This calculator applies these standard IVF dating formulas precisely, which often produces a due date a few days different from natural conception calculators using the same transfer date.
What is the implantation window and when does it happen?
Implantation is the process by which the fertilized egg (now a blastocyst) attaches to the uterine lining, where it will develop into a placenta and embryo. Implantation does not happen immediately after fertilization. The fertilized egg spends approximately 5 to 6 days traveling through the fallopian tube to the uterus, and then takes several more days to implant. The implantation window spans from 6 to 12 days after ovulation, with the highest probability occurring at 9 days post-ovulation (approximately 34% of successful pregnancies implant on this day). Once implantation is complete, the developing embryo begins producing hCG (human chorionic gonadotropin), the hormone detected by pregnancy tests. Blood tests can detect hCG 3 to 4 days after implantation; urine home pregnancy tests typically detect it 1 to 2 days later. The earliest reliable positive home test is therefore approximately 14 days after ovulation, often called 14 DPO (days post-ovulation).