Estimate your conception date window, fertile window and due date based on LMP, due date or ultrasound.
When exactly did I get pregnant? Many women ask this question when trying to figure out the exact day their pregnancy began. Often, gestational age, or the age of the baby, is calculated from the first day of the mother's last menstrual period. Biologically, the baby was not conceived until ovulation and the fertilization of the egg, which usually happens at least 10 days after the first day of the mother's last menstrual period.
Because there are many factors surrounding conception, it is difficult to pinpoint an exact date of conception. This calculator helps to estimate the date range of real conception and a possible range of days during which sexual intercourse might have led to conception.
Conception usually occurs around 11-21 days after the first day of the last period of a woman who has a regular period. The estimation of conception date is based on this, but is rarely ever exact since it is difficult to know exactly when ovulation occurs. Estimating conception date can also be more difficult for women who have irregular periods, or for those who don't remember the first day of their last period.
An estimated due date, usually based on a sonogram, is typically obtained from a healthcare provider during a prenatal visit. Based on this due date, a range of possible conception dates can be calculated by reverse-engineering the standard 280-day pregnancy timeline.
An ultrasound, on a basic level, uses sound waves to create images of internal body structures. An ultrasound used in the context of examining the progress of the fetus in pregnant women is referred to as an "obstetric ultrasound." This is done in real-time and is a fairly standard procedure that is part of prenatal care in many countries.
Ultrasounds can be used to measure a fetus as early as five or six weeks following the mother's last menstrual period. It is a more accurate measure in early pregnancy than it is in later stages. Ultrasound can be used in conjunction with estimates based on the last menstrual period to determine a more accurate gestational age.
Fertilization and conception are terms that are often used interchangeably. This calculator uses the medical and scientific definition of the terms. Fertilization occurs when the sperm and the egg combine in the fallopian tube, producing a fertilized egg, and beginning the process in which an embryo develops.
Conception refers to the process of becoming pregnant, which scientifically involves both fertilization as well as implantation into the wall of the uterus. On the state level (in the U.S.) there is some confusion between the use of the terms conception and fertilization due to differences in the definitions used.
The long-standing view of those in the medical profession, as well as federal policy (in the U.S.) considers a woman pregnant when a fertilized egg has implanted into the wall of her uterus. When this has occurred, the woman is said to have conceived. In other words, conception, medically and scientifically, requires both fertilization and implantation, and a woman is not considered pregnant until both have occurred.
| Event | Typical Timing (from LMP) | Description |
|---|---|---|
| Last Menstrual Period | Day 0 | First day of last period; standard starting point for gestational age |
| Fertile Window | Days 9 – 15 | Days when intercourse is most likely to result in pregnancy (sperm can survive 3-5 days) |
| Ovulation | Day 14 (average) | Release of mature egg from ovary |
| Fertilization | Days 12 – 16 | Sperm penetrates egg in fallopian tube |
| Implantation | Days 20 – 24 | Embryo attaches to uterine wall (official start of pregnancy) |
| Due Date | Day 280 | Estimated date of delivery (40 weeks from LMP) |
All dates provided by this calculator are estimates based on average 28-day menstrual cycles and standard 40-week pregnancy terms. Actual conception dates can vary significantly due to irregular cycles, late ovulation, implantation timing and individual biological variation. First-trimester ultrasounds are generally considered the most accurate method for dating pregnancy. This calculator is for informational purposes only and does not constitute medical advice. Always consult with a qualified healthcare provider for confirmation of pregnancy dating and prenatal care.