Eli5: Why is pregnancy counted from the first day of your last period?
In: Biology
Your last period started a new cycle, you released an egg, it got fertilized with a man’s sperm and now you’re pregnant.
Ovulation usually happens 2ish weeks after your period, but that’s not a hard guarantee by any means even if your cycle is pretty regular.
Since you don’t know exactly when you ovulated, the count starts from your last period for simplicity’s sake.
Alright, so imagine this: pregnancy is like planning a big party. You need to know when the prep started to estimate when the party will happen. Now, a woman’s body is super complex, and figuring out exactly when the egg got fertilized (like pinpointing the exact moment a party invitation was accepted) is really tricky. But, there’s a clear and obvious start to the whole process: the first day of the last period. This is like saying, “Okay, the party planning started on this date.”
Doctors use this date because it’s a solid, clear starting point that women can easily track. The actual fertilization (when the sperm meets the egg) happens about two weeks after that, but since everyone’s cycles are a bit different, counting from the last period gives a consistent starting point to estimate the due date.
So, it’s like saying, “Let’s count from when we started planning the party, not from when we sent the invitation.” It’s all about having a reliable starting point to work from
Because every woman has a different period cycle so there’s no way for know exactly when she ovulated, let alone when the sperm intercepted the egg. The day of the last menstrual period (LMP) is the only thing people can definitively say. Later as the pregnancy progresses, things such as hormone levels or dating ultrasounds (different organs develop in different weeks) can be used to guess-timate hold old the fetus is. This can be helpful if the woman forgot the date of the LMP or has irregular or unusually short/long menstrual cycles
What everyone else said.
And probably because the system is from before hormone tests and ultrasound so you were going with the first clear sign, and it works well enough there is no reason to change.
Latest Answers