RoyalCalculator
Work out roughly when conception happened — and the window of days intercourse could have led to it, since sperm can survive in the body for a few days before ovulation — from your due date, last period, or an ultrasound date.
It's a question a lot of newly pregnant women ask, and it's genuinely hard to answer precisely. Gestational age — how far along a pregnancy is — is conventionally counted from the first day of the last menstrual period (LMP), but conception itself doesn't happen until later: ovulation and fertilization of the egg, which typically occurs at least 10 days after that LMP date. Because ovulation timing varies even cycle to cycle in the same person, pinning down an exact date is rarely possible — this calculator estimates a range instead of a single definitive day.
Last menstrual period — For someone with regular cycles, ovulation (and so conception) generally falls somewhere between 11 and 21 days after the LMP. This is the most common starting point, but it's inherently approximate, and considerably less reliable for anyone with irregular cycles or who isn't sure of their last period's start date.
Due date — A due date set by a healthcare provider, usually from a sonogram, can be worked backward to the same conception window, since due dates and LMP-based estimates are built on the same underlying timeline.
Ultrasound — An obstetric ultrasound measures the fetus directly using sound waves, and can be used from as early as five or six weeks after the LMP. It's most accurate earlier in pregnancy, since growth rates become less uniform later on, and is often combined with an LMP-based estimate to refine the overall gestational age.
These two words get used as if they're interchangeable, but medically they're not. Fertilization is specifically the moment sperm and egg combine in the fallopian tube, kicking off embryo development. Conception is broader — it requires both that fertilization and the embryo's subsequent implantation into the uterine wall. The established medical and legal position (including U.S. federal policy) is that pregnancy begins at implantation, not at fertilization, even though the two terms sometimes get conflated in casual or legal discussion at the state level. Until implantation occurs, a woman is not considered, medically, to be pregnant.