To understand pregnancy, first we have to talk about ovulation. Once a month, an egg leaves the ovary. This is called ovulation and it occurs about halfway through your menstrual cycle. After the egg leaves the ovary, it travels through the fallopian tube, which takes about 12 to 24 hours. If there is semen in the vagina, the sperm cells will swim up the fallopian tubes, looking for an egg. If a sperm cell meets an egg, that egg becomes fertilised, at which point it's called a zygote. According to Planned Parenthood, sperm cells can live inside the body for up to six days, so you don’t have to have sex on the same day you ovulate to become pregnant.
A fertilised egg isn't the same thing as a pregnancy. First, the zygote divides into more and more cells, at which point it’s called a blastocyst or a pre-embryo. The blastocyst arrives at the uterus three to four days after fertilisation. Then, it hangs out in the uterus for another two or three days. If it attaches to your uterine lining, this is called implantation, and it’s considered to be the beginning of pregnancy. The implantation process can take, you guessed it, another three or four days. If the fertilised egg doesn’t implant, it will leave your body during your period. This happens to about half of all fertilised eggs that make it to the uterus.
All these days add up. According to Planned Parenthood, it can take up to two or three weeks between having sex and actually being pregnant. To make it more complicated, doctors measure pregnancy from the first day of your last period — even though you weren't technically pregnant yet.
Now, how soon can you know that you’re pregnant? When the fertilised egg implants, your body begins releasing pregnancy hormones called human chorionic gonadotropin (hCG). These hormones prevent your uterine lining from shedding, which is why you don’t have a period when you’re pregnant. These hormones are also what pregnancy tests measure.
Different types of pregnancy tests detect different levels of hCG. “If intercourse happens around the time of ovulation, then approximately 14 days later, a positive urine home pregnancy test should be detectable,” Shahin Ghadir, M.D., F.A.C.O.G., tells Refinery29. While some tests may work slightly earlier, usually, you'll want to wait to take an over-the-counter pregnancy test the day after your missed period to avoid the risk of a false negative. A blood test from a doctor can detect pregnancy earlier, about ten days after ovulation, or a week before your period is due.
If you’re conceiving through in vitro fertilisation (IVF), things work a little differently. The egg is fertilised outside your body (that’s where the term “in vitro” comes from), and then a doctor puts the fertilised egg or eggs directly in the uterus in a process called an embryo transfer. If any of the fertilised eggs implant (which again, can take a couple days), that’s when pregnancy begins. However, many people receive a dose of hCG as part of the IVF process, so at-home pregnancy tests may not be accurate. “Generally, a blood test will work ten days after the embryo transfer into the uterus,” Dr. Nadir says.