Conception Date Calculator
Calculate your estimated conception date from your last menstrual period or due date. Free online conception calculator with gestational age, trimester, and pregnancy timeline.
About This Calculator
The Conception Date Calculator helps you estimate when you conceived your baby based on the first day of your last menstrual period (LMP) or your estimated due date. Whether you have just found out you are pregnant and want to know when conception occurred, or you are planning ahead and want to understand your fertility timeline, this calculator provides instant, medically-informed results.
Conception occurs when a sperm fertilises an egg during ovulation. For a typical 28-day menstrual cycle, ovulation happens around day 14 after the first day of your LMP. The calculator uses Naegele's rule and the standard ovulation model: conception date is estimated as LMP + (cycle length − 14) days. If you provide your due date, we calculate backwards by subtracting 266 days (the typical length of pregnancy from conception). Your due date is calculated as LMP + 280 days (40 weeks).
Your gestational age (how far along you are in your pregnancy) is calculated from your LMP to today, divided into weeks and days. The calculator also identifies which trimester you are in: First Trimester (weeks 1-13), Second Trimester (weeks 14-27), or Third Trimester (weeks 28-40). The pregnancy timeline chart visualises your progress week by week throughout the 40-week journey.
Regional Notes
India: The conception date and due date estimates follow the same medical standards globally. Indian obstetricians typically use LMP-based dating with ultrasound confirmation. The full pregnancy term is 40 weeks (280 days) from LMP regardless of region.
United States: ACOG (American College of Obstetricians and Gynecologists) recommends ultrasound dating in the first trimester for the most accurate gestational age assessment. The calculator's estimates align with ACOG guidelines using Naegele's rule.
United Kingdom: NICE guidelines recommend a dating ultrasound scan between 10 and 14 weeks. The calculated estimates here serve as a preliminary guide — always confirm with your midwife or obstetrician through ultrasound dating.
Frequently Asked Questions
How is the conception date calculated?
The conception date is estimated by calculating 14 days before your next expected period. For a 28-day cycle, ovulation and conception typically occur around day 14 after the first day of your last menstrual period (LMP). If using your due date, we subtract 266 days (38 weeks) to estimate the conception date.
When did I conceive based on my due date?
To find your conception date from your due date, subtract 266 days (38 weeks) from your estimated due date. For example, if your due date is September 23, 2024, your estimated conception date would be January 1, 2024. This assumes a standard 28-day menstrual cycle.
Can I calculate my due date from my conception date?
Yes, pregnancy typically lasts about 266 days (38 weeks) from conception. To estimate your due date from conception, simply add 266 days to your conception date. Alternatively, you can enter your LMP and the calculator will compute both conception date and due date automatically.
How accurate is the conception date calculator?
No conception date calculator can be 100% accurate since conception usually occurs within a fertile window of about 6 days (5 days before ovulation plus ovulation day). The calculator provides an estimate based on standard medical formulas — Naegele's rule for due dates and the ovulation model for conception. Ultrasound dating in early pregnancy is more accurate.
What is the difference between LMP and conception date?
The Last Menstrual Period (LMP) is the first day of your last menstrual bleeding. Conception (fertilization) typically occurs about 14 days after LMP in a standard 28-day cycle. Pregnancy is measured from LMP, which adds about 2 weeks before conception actually occurred. So at 4 weeks pregnant, conception was approximately 2 weeks ago.
What if my cycle is longer or shorter than 28 days?
The calculator allows you to enter your average cycle length between 20 and 45 days. For longer cycles, ovulation occurs later (e.g., day 17 for a 31-day cycle), and for shorter cycles, ovulation occurs earlier (e.g., day 10 for a 24-day cycle). The formula adjusts the conception date accordingly: LMP + (cycle length - 14) days.
How many weeks pregnant am I based on conception date?
Pregnancy is typically measured from LMP, not conception date. Your gestational age in weeks is calculated from the first day of your LMP to today. At 40 weeks from LMP, you are about 38 weeks from conception. The calculator shows both your gestational age and your days pregnant for clarity.
What trimester am I in?
Pregnancy is divided into three trimesters: First Trimester (weeks 1-13), Second Trimester (weeks 14-27), and Third Trimester (weeks 28-40). The calculator automatically shows your current trimester based on your gestational age calculated from your LMP or due date.