Estimate the date you conceived and your likely intercourse fertile window based on LMP, expected due date, or ultrasound scans.
When Did I Conceive?
Pinpointing the exact day of conception is challenging due to the natural variation in ovulation timing. Gestational age is typically counted starting from the first day of the mother's Last Menstrual Period (LMP), which means you are technically considered "pregnant" about two weeks before the baby is actually conceived.
This calculator estimates the probable date of conception and identifies the possible fertile window—the days when sexual intercourse is most likely to result in pregnancy, based on sperm remaining viable inside the body for 3 to 5 days.
Conception Dating Methods
In regular 28-day cycles, ovulation and conception generally happen 11 to 21 days after the first day of the last period. Irregular cycles can widen this window considerably.
Based on the standard 266-day post-fertilization gestation period, the conception date is estimated by subtracting 266 days (or 38 weeks) from the expected due date provided by your doctor.
Obstetric ultrasounds use high-frequency sound waves to measure fetal structures. Scanning in the first trimester (weeks 6–12) is the most accurate clinical method for estimating gestational age.
Fertilization, Conception, and Pregnancy (Scientific Definitions)
Although often used interchangeably in everyday conversation, medical and scientific bodies distinguish the stages of early pregnancy:
• Fertilization occurs when a single sperm joins an egg cell in the fallopian tube, creating a zygote and initiating cell division.
• Conception is the broader biological process of becoming pregnant. Medically, this requires both fertilization and successful **implantation** of the blastocyst/embryo into the lining of the uterus.
• Medical Pregnancy officially begins at the point of implantation (not fertilization). This standard definition is shared by major medical organizations, including the American College of Obstetricians and Gynecologists (ACOG), as well as federal health policy guidelines.
How it Works & Formula
Estimates when conception likely occurred based on current due date or last menstrual period date.
Practical Examples
For a due date of May 15th, conception likely happened between August 20th and August 25th of the previous year.
Frequently Asked Questions
When exactly did I get pregnant?
Biological conception occurs when a sperm fertilizes an egg, typically around ovulation. This happens about 11–21 days after the first day of your last menstrual period (LMP) for regular cycles.
How does the calculator estimate the sexual intercourse window?
Sperm can remain viable inside the female reproductive tract for up to 3 to 5 days. Therefore, sex occurring up to 5 days before ovulation/conception and up to 1 day after can result in pregnancy.
Which conception calculation method is most accurate?
Early obstetric ultrasound scans (performed in the first trimester between 6 and 12 weeks) are generally considered the most accurate method for estimating gestational age and back-calculating conception.