Pregnancy Conception Calculator

Pregnancy is dated from the first day of the last menstrual period (LMP), about two weeks before conception actually happens. This calculator reverses the usual due date math: it finds the LMP that matches your information, adds two weeks to estimate ovulation and conception, and shows the days on which intercourse was most likely to have led to the pregnancy.

When Did I Conceive?

In a textbook 28-day cycle, ovulation happens about 14 days after a period starts, and an egg can be fertilized for only about 12 to 24 hours after it is released. Conception therefore usually takes place within a day of ovulation. Because the standard due date is 280 days after the LMP, conception is estimated at 266 days, or 38 weeks, before the due date.

The date of intercourse is a different question. Sperm can survive in the reproductive tract for up to five days, so sex on any of the five days before ovulation, or on the day itself, can result in pregnancy. That is why the calculator shows a six-day fertile window alongside the single most likely conception date. Pinpointing one act of intercourse from a due date is generally not possible.

Cycle length shifts the timing. The phase after ovulation, the luteal phase, is fairly constant at about 14 days, so most of the variation in cycle length happens before ovulation. Someone with a 32-day cycle typically ovulates around day 18, four days later than in a 28-day cycle. When you choose the last period method, the calculator adjusts for your average cycle length.

An early ultrasound is usually the most accurate way to date a pregnancy. In the first trimester, the length of the embryo from crown to rump closely tracks age, and measurements before about 14 weeks are typically accurate to within five to seven days. Later scans become less precise because babies grow at different rates. If your provider changed your due date after an ultrasound, use the updated date.

Even with careful dating, the estimate is only that. Ovulation can come early or late in any given cycle, and only about 4% of babies are born on their due date. The calculator gives the most likely timing, but a range of several days either way is normal.

Fertilization, Conception, and Pregnancy

These words are often used interchangeably, but they describe different steps. Fertilization is the moment a sperm joins an egg, usually in the fallopian tube. The fertilized egg, called a zygote, begins dividing as it travels toward the uterus over the next several days. Conception is commonly used to mean fertilization, though some definitions include implantation.

Around five to six days after fertilization, the embryo has become a blastocyst and begins to attach to the lining of the uterus. Implantation is usually complete within about 10 days of ovulation. Only then does the body start producing enough human chorionic gonadotropin (hCG), the hormone that pregnancy tests detect. Medically, pregnancy is considered established once implantation has occurred.

Frequently asked questions

Sources

• ACOG – Methods for Estimating the Due Date
• NIH NICHD – Pregnancy
• Office on Women's Health – Pregnancy