Pregnancy Conception Calculator

When did I conceive?

Pregnancy is dated from the first day of the last menstrual period, but biologically the baby was not conceived until ovulation and fertilization — usually at least 10 days after that first day. Because ovulation timing varies and sperm survive for days, an exact conception day is rarely knowable; what can be estimated is a window. This calculator builds a "most probable" window around the nominal conception date implied by your input, and a wider "possible" window around that, then shifts the start of each window back to cover the days intercourse could have led to it.

Last menstrual period. With a regular cycle, conception typically falls about 11–21 days after the period starts — which is why the cycle-length setting matters: ovulation sits roughly 14 days before the next period, so longer cycles conceive later. The estimate weakens with irregular cycles or an uncertain period date; in those cases an ultrasound is the better anchor. Due date. A due date from a prenatal visit (usually set by sonogram) counts back 266 days to a nominal conception. Ultrasound. An obstetric ultrasound images the fetus directly; it can measure one as early as five or six weeks after the last period, is most accurate early in pregnancy, and combines well with period-based estimates to pin down gestational age.

Fertilization, conception, and pregnancy

The words are often used interchangeably, but medically they are distinct steps. Fertilization is the moment sperm and egg combine in the fallopian tube, producing a fertilized egg from which an embryo develops. Conception, in the long-standing medical and scientific definition, is the whole process of becoming pregnant: fertilization plus implantation of the fertilized egg into the wall of the uterus. A woman is considered pregnant once implantation has occurred — which is also why some public and state-level usage, which treats fertilization alone as the start, differs from the medical definition this calculator uses.