When Did I Conceive?

When did I conceive? Many women wonder the date their pregnancy started. In practice, gestational age, or the age of the fetus, is determined from the first day of the last menstrual period. Biologically, the fetus was not conceived until ovulation and fertilization of the egg, which usually happens at least 10 days after the first day of the last menstrual period. There are so many variables surrounding the date of conception, that determining the exact date of conception is nearly impossible. The calculator can estimate the approximate date ranges of actual conception, and possible days of sexual intercourse that could have resulted in conception. There are various methods available to estimate conception dates.

Last Menstrual Period

Typically, conception takes place between 11-21 days after the first day of a woman’s last period when she has a regular cycle. We estimate conception date using this information, but it is hardly ever perfect, since the timing of ovulation is almost impossible to pinpoint the exact day. It can be even harder to estimate conception date in women who have unstable cycles, or do not recall when their last period began. In these cases, one of the more accurate methods of estimating gestational age is through an ultrasound.

Due Date

Typically, a healthcare provider will establish an estimated due date which is sometimes based on a sonogram, during a prenatal appointment. From the established due date, we can also infer a possible range of conception dates.

Ultrasound

At its most basic level, ultrasound uses sound waves to generate pictures of internal body parts. An ultrasound, used in a care situation to view the development of the fetus in pregnant women, would be called an “obstetric ultrasound.” This is accomplished in real time and is a fairly common standard of care in many countries as a part of prenatal care, as it provides information about the mother’s health, embryo/fetus health, and the overall development and timing of the pregnancy. Ultrasounds can measure a fetus as early as five or six weeks from the mother’s last menstrual period, and is much more accurate if measuring early in pregnancy than later in pregnancy. Ultrasound can also run concurrent with estimations based on the last menstrual period to provide a more accurate gestational age estimation.

Fertilization, Conception, and Pregnancy

Fertilization and conception are terms that are often used synonymously. This calculator defines the terms as they are described in the medical and scientific literature. Fertilization occurs when the sperm and the egg join in the fallopian tube to produce a fertilized egg and initiate the process that eventually leads to the development of an embryo.

Conception, however, refers to getting pregnant, which, by biological definition includes fertilization and implantation of the fertilized egg into the wall of the uterus. There has been some confusion across state lines (in the U.S.) or state laws differentiating between fertilization and conception due to the different definitions used to describe those terms. In some cases, a woman has been declared pregnant after fertilization, which is inconsistent with accepted definitions in medical and scientific literature. The long-standing standards in the medical field aligns with federal law (i.e., the U.S.) that considers a woman pregnant after a fertilized egg has implanted in the wall of the uterus. Once implantation has occurred, the woman is said to have conceived. In other words, conception in the medical or scientific understanding includes both fertilization and implantation into the uterus, therefore a woman will not be considered pregnant without implantation of a fertilized egg into the uterus.