Lauren Guy wrote a fantastic piece about why we need to avoid the “pregnant two weeks before you are pregnant,” talking point when it comes to the Arizona 18-weeks post-conception abortion ban, but even in her effort to clarify gestational dating, some  details still aren’t quite correct.

Guy states:

First of all, from a health care standpoint, pregnancy is nearly always measured from two weeks prior to when implantation (the physiological beginning of pregnancy) actually occurs.  All clinicians – doctors, midwives, the good folks at your local Planned Parenthood–measure pregnancy not from how long a person has actually been pregnant, but from their last menstrual period (LMP), which falls about two weeks prior to implantation. This is partly because we don’t always know exactly when a person actually became pregnant, but mostly because it’s far more accurate to measure a person’s unique gestation rate by her cycle.

But that’s not technically correct. Using LMP as a gauge for gestational age is the correct, clinical measure for dating a pregnancy. LMP, however, is traditionally two weeks prior to fertilization, not implantation, an act that occurs later in the cycle.

In a “typical” 28 days cycle, Day 1 is the first day of your period. A standard medical cycle would then have a woman ovulating at roughly day 14–two weeks into the cycle.  At that point the egg is released and sloughed off through the menstrual cycle; may be fertilized by a sperm already there, or in some cases become fertilized by new sperm that day or the day following (an egg can generally be fertilized for about 24 hours). Before pregnancy can be established, a fertilized egg must travel down the fallopian tube and implant in the uterine lining, dividing as it goes. The process takes in general 7 days, although sometimes it can be a few days more or less. Only once the egg embeds in the uterus does it begin creating the hormone for which pregnancy tests are used to detect a pregnancy.

Although some women who track their fertility cycles (temperatures, cervical mucus, etc.) can often detect ovulation within a matter of a day, there is no way to tell exactly when or if an egg implants (although for those of us who have tracked, man, we sure wish we could). By 8 days past ovulation (dpo) if implantation has occurred a woman can possibly begin testing to see if she is or isn’t pregnant, although a reliable answer can  usually be gotten around 10-12 dpo at the earliest.

So, for dating purposes, pregnancy begins at 2 weeks before ovulation. Implantation occurs roughly 3 weeks into the cycle if a woman gets pregnant, and a period is missed or test shows positive within the week following that. Or, she will menstruate, making her once again at cycle day 1 and 2 weeks prior to pregnancy.