When Did I Get Pregnant?
Our Conception Date Calculator uses four clinically informed methods to estimate when conception likely occurred. Enter your due date, last menstrual period, ultrasound data, or your baby's date of birth to find your conception window, gestational age, and pregnancy milestones. All calculations follow standard obstetric formulas referenced by ACOG (American College of Obstetricians and Gynecologists) and Mayo Clinic guidelines.
Calculate by Last Menstrual Period (LMP)
The LMP method is the most widely used first step in pregnancy dating. It applies Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period to estimate your due date. Conception is then estimated at approximately LMP + 14 days, assuming a standard 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the calculator adjusts the ovulation estimate accordingly. This method is most reliable for women with regular cycles and is typically used before 12 weeks of gestation, according to ACOG clinical guidelines.
Key Parameters:
- Standard 28-day cycle assumption (adjustable 20-45 days)
- Ovulation estimated at cycle length minus 14 days
- Accuracy: best for regular cycles, ±2 weeks for irregular cycles
Calculate by Due Date
If your healthcare provider has given you a confirmed due date (often based on a first-trimester ultrasound), you can reverse-calculate your conception date by subtracting 266 days (38 weeks) from that date. This is one of the most straightforward methods and works well when the due date has been verified by a medical professional. The calculator also shows your conception window and pregnancy milestones relative to that date.
Key Parameters:
- Reverse calculation: Due Date - 266 days = Estimated Conception
- Accuracy depends on how the due date was originally determined
- Most accurate when due date is based on early ultrasound
Calculate by Ultrasound Date
Ultrasound-based dating is the most accurate method, especially in the first trimester. The calculator uses your scan date, the gestational age reported at the scan, and optional fetal measurements (CRL, BPD, or FL) to estimate the conception date. According to the NHS and ACOG, first-trimester ultrasounds are accurate to within ±5-7 days, making this the gold standard for pregnancy dating when LMP is uncertain or the cycle is irregular.
Precision by Trimester:
- First trimester (before 12 weeks): ±5-7 days
- Second trimester (13-20 weeks): ±10-14 days
- Third trimester (after 28 weeks): ±21-30 days
Calculate by Date of Birth (Reverse Calculator)
After your baby is born, you can reverse-calculate when conception likely occurred. This method subtracts 266 days (38 weeks) from the baby's date of birth to estimate the conception date for a full-term delivery. If the baby was born prematurely, the calculator adjusts the gestation length by the number of weeks early (e.g., 4 weeks premature means a gestation of 266 - 28 = 238 days). The result includes a conception window of ±7 days to reflect the inherent uncertainty in this reverse calculation.
This method is useful for understanding your pregnancy timeline after delivery, answering paternity questions, completing medical records, or simply satisfying personal curiosity about when your baby was conceived. It is the least precise of the four methods, since it does not account for individual variation in gestation length, but it provides a reasonable estimate when no other data is available.
Formula:
- Full-term: Conception Date ≈ Date of Birth - 266 days
- Premature: Conception Date ≈ Date of Birth - (266 - premature weeks × 7) days
- Conception window: estimated date ±7 days
- Accuracy: ±2-3 weeks
How to Calculate Your Conception Date
What Is a Conception Date?
A conception date is the estimated date when a sperm cell fertilized an egg, typically occurring within 12-24 hours after ovulation. It is important to distinguish this from the gestational age start date, which doctors count from the first day of the last menstrual period (LMP) — approximately two weeks before conception actually takes place. This distinction matters because prenatal testing schedules, developmental milestones, and due date calculations all depend on whether gestational age or fetal age is being referenced. Understanding your conception date helps with due date accuracy, timing of prenatal tests, and in some cases, paternity determination.
Naegele's Rule and the 266-Day Formula
Naegele's rule, developed by German obstetrician Franz Naegele in the early 1800s, is the standard clinical method for estimating a due date. The rule states: Due Date = LMP + 280 days (40 weeks). Since conception typically occurs around day 14 of a 28-day cycle, the reverse formula is: Conception Date ≈ Due Date - 266 days (or equivalently, LMP + 14 days).
Example Calculation:
- LMP: January 1, 2025
- Estimated Ovulation: January 1 + 14 days = January 15, 2025
- Estimated Conception: ~January 15, 2025
- Estimated Due Date: January 1 + 280 days = October 8, 2025
- Verification: October 8 - 266 days = ~January 15 (matches)
For women with longer cycles (e.g., 35 days), ovulation occurs later (around day 21), so the conception date shifts forward by about one week. The calculator handles this adjustment automatically when you enter your cycle length.
The Conception Window: Why It's a Range, Not a Single Day
The calculator displays a conception window rather than a single date because pinpointing the exact moment of conception is virtually impossible. Sperm can survive in the female reproductive tract for 3-5 days, meaning intercourse several days before ovulation can still result in conception. Meanwhile, the released egg remains viable for only 12-24 hours. This creates a fertile window of approximately 6 days: the 5 days before ovulation plus ovulation day itself. Because of this biology, the calculator shows a date range to reflect the realistic window during which conception most likely occurred.
Sperm Survival and the Fertility Window
According to the Mayo Clinic and ACOG, sperm can survive inside the reproductive tract for up to 5 days under optimal conditions, though 3 days is more typical. The egg is viable for only 12-24 hours after release from the ovary. This means the most fertile period is the 2-3 days leading up to ovulation and ovulation day itself. Intercourse even 4-5 days before ovulation can result in pregnancy if sperm survive long enough. This is why our calculator shows both a "most probable" and a "possible" conception range — the narrower range covers the highest-probability days, while the wider range accounts for the full extent of sperm viability.
Gestational Age vs Fetal Age Explained
One of the most common sources of confusion in pregnancy is the difference between gestational age and fetal age(also called embryonic age or conceptional age). Gestational age is counted from the first day of the last menstrual period, which is approximately 2 weeks before conception. Fetal age is counted from the actual date of fertilization. This means that at "10 weeks pregnant" (gestational age), the fetus is actually about 8 weeks old (fetal age).
Why Doctors Count From Your Last Period
Doctors use gestational age as the standard measurement because the first day of the last menstrual period is a date most women can identify with certainty, whereas the exact day of ovulation and conception is rarely known. This convention has been in use for over 200 years (since Naegele's time) and is the basis for all prenatal milestone charts, screening test windows, and delivery planning. When your doctor says you are "12 weeks pregnant," they mean 12 weeks gestational age — the fetus itself is approximately 10 weeks old. This 2-week offset matters for developmental milestones: organ development timelines, anatomy scan scheduling (typically at 18-20 weeks gestational, or 16-18 weeks fetal), and viability assessments are all calibrated to gestational age.
How Accurate Is a Conception Calculator?
Accuracy by Method (LMP vs Ultrasound vs Due Date)
- LMP method: ±2 weeks, assuming a regular 28-day cycle. Less accurate for irregular cycles.
- Ultrasound method: ±5-7 days in the first trimester (gold standard), ±10-14 days in the second trimester, ±21 days in the third.
- Due date method: accuracy mirrors whatever method was used to set the due date. If it was based on an early ultrasound, accuracy is ±5-7 days.
- Date of birth method: ±2-3 weeks, since it assumes standard gestation length and cannot account for natural variation.
Factors That Affect Accuracy (Irregular Cycles, Late Ovulation)
Key Factors:
- Irregular menstrual cycles: if your cycle length varies significantly, LMP-based calculations may be off by 1-2 weeks or more.
- Late or early ovulation: women with longer cycles ovulate later; women with shorter cycles ovulate earlier. The standard day-14 assumption does not apply to everyone.
- Implantation timing: after fertilization, the embryo takes 6-12 days to implant, which can affect early pregnancy hormone levels and detection.
- Multiple pregnancies: twins and higher-order multiples may have different growth patterns that affect ultrasound dating accuracy.
- Previous pregnancy history: women who have had previous pregnancies may have slightly different gestation lengths.
Can Conception Date Help Determine Paternity?
A conception date calculator can narrow down the window during which conception occurred to approximately 5-7 days. By comparing this window with the dates of intercourse, it may help indicate the likely biological father. However, it is important to understand the limitations: the conception window is an estimate, and the margin of error increases with irregular cycles or uncertain LMP dates. Sperm can survive up to 5 days, meaning intercourse several days before ovulation could be the source of conception. Only a DNA paternity test can provide definitive, legally admissible results. If paternity is in question, we recommend consulting a healthcare provider or a certified genetic testing service for accurate determination.
Want to learn more about how conception dates are calculated and what they mean? Read our in-depth guide: When Did I Conceive? How to Calculate Your Conception Date. You may also find our date calculator useful for general date arithmetic.