When trying to discuss how far along someone is in pregnancy, you could use a variety of terms — trimesters, months, and weeks are all ways women and their doctors measure pregnancies. But, when does pregnancy actually start? Does your pregnancy start from your last period? It can be hard to actually know the definite day you become pregnant, so the medical world has assumed some commonalities when measuring pregnancies.
According to the American Pregnancy Association (APA), the development of pregnancy is assumed to begin from the first day of your last menstrual period. This is the case even though the development of the fetus doesn't actually begin until conception, which is typically a couple of weeks later. So, as Fit Pregnancy mentioned, though pregnancies are said to be about 40 weeks, you only really carry your baby for about 38 weeks.
As the APA noted, the reason pregnancy is counted this way is because with each menstrual period, your body is actually preparing for a baby. Since it's hard to know when you actually conceive, it's easier for doctors to use a universal method of measurement.
The first week of your menstrual cycle is when you have your period. The next week, your uterine lining begins to thicken in order to prepare for a fertilized egg, and then, ovulation occurs. If that eggs gets fertilized, your pregnancy actually begins in week three, according to the Mayo Clinic.
You may not actually know you're pregnant until well into week four, as the most common sign of pregnancy is a missed period, Fit Pregnancy noted. You may also notice yourself peeing more, feeling extra tired, or having sore breasts.
While your pregnancy may not technically start until around week three of your cycle, if you are trying to get pregnant, staying healthy is always a good idea. Taking vitamins, getting enough rest, and drinking plenty of water is a great pregnancy habit to get into right away. It can be confusing to calculate when you actually became pregnant, but the countdown to the big day will be worth it.