Why do we calculate the length of a pregnancy from the first day of a women’s last period?
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I’m not a doctor, but I am pregnant! My due date was based on the first day of my last period until my first ultrasound.
This is because it’s difficult to know exactly when you ovulate & conception occurs. I tracked my ovulation so we actually did know, but then you also have to consider the implantation process.
The embryo does not begin dividing cells and growing until it is implanted in the uterus. After that, a pregnancy test will show up positive.
Using your last period won’t always be the most accurate, but it is the easiest to track! Plus, due dates can always be changed after a dating scan if need be.
Fun fact: I don’t know if it’s the same everywhere, but with animals you count the gestation time from the day of conception. Probably because you can exactly say when the day of conception was, whereas with humans there probably is multiple times in fertility time where people have sex.
Before ultrasounds existed, it was the best way to get a theoretical date of conception. From their calculations in the early 1900s, the average woman ovulated on Day 14 of her cycle. The cycle starts the first date of her last menstrual period (LMP). The child cannot be conceived prior to the woman ovulating, so identifying when she ovulated is key to understanding when she conceived. Since the date of ovulation/conception is calculated from the LMP, they just run with that.
It’s a little silly because when you are four weeks pregnant, the zygote/embryo has only existed for two weeks. It’s basically a historical artifact from a time when we knew very little about women’s fertility.
We have now studied female anatomy enough to know that this is only a very rough estimate. Women don’t necessarily ovulate on day 14. Women may have long or irregular cycles. If you live in a developed country, they will often revise the expected due date based on an ultrasound or on urine/blood testing if they look more than one week off. I use Natural Family Planning to track when I actually ovulate and my doctors/midwives have been willing to revise my due date based on that – so long as the other data fits.
Fun fact: I don’t know if it’s the same everywhere, but with animals you count the gestation time from the day of conception. Probably because you can exactly say when the day of conception was, whereas with humans there probably is multiple times in fertility time where people have sex.
Before ultrasounds existed, it was the best way to get a theoretical date of conception. From their calculations in the early 1900s, the average woman ovulated on Day 14 of her cycle. The cycle starts the first date of her last menstrual period (LMP). The child cannot be conceived prior to the woman ovulating, so identifying when she ovulated is key to understanding when she conceived. Since the date of ovulation/conception is calculated from the LMP, they just run with that.
It’s a little silly because when you are four weeks pregnant, the zygote/embryo has only existed for two weeks. It’s basically a historical artifact from a time when we knew very little about women’s fertility.
We have now studied female anatomy enough to know that this is only a very rough estimate. Women don’t necessarily ovulate on day 14. Women may have long or irregular cycles. If you live in a developed country, they will often revise the expected due date based on an ultrasound or on urine/blood testing if they look more than one week off. I use Natural Family Planning to track when I actually ovulate and my doctors/midwives have been willing to revise my due date based on that – so long as the other data fits.
There’s some talk of “we don’t know when a woman ovulated” but we do know that for most women who track their periods, their ovulations, and/or are trying to conceive.
Like, my wife knows the exact day she ovulated for the kid we had. But they still go off the last period, and doctors still assume cycles are 28 days or say “it varies” even when you know how long your cycle is.
There’s some talk of “we don’t know when a woman ovulated” but we do know that for most women who track their periods, their ovulations, and/or are trying to conceive.
Like, my wife knows the exact day she ovulated for the kid we had. But they still go off the last period, and doctors still assume cycles are 28 days or say “it varies” even when you know how long your cycle is.
Because you could have gotten pregnant at any time after that period ended. You’re technically more likely to get pregnant during ovulation but there have been cases of getting pregnant when the last time she had sex was in the middle of her period because the Soren survived long enough.
TLDR the last day of your last period is the last day you can *guarantee* that you weren’t pregnant.
Before ultrasounds existed, it was the best way to get a theoretical date of conception. From their calculations in the early 1900s, the average woman ovulated on Day 14 of her cycle. The cycle starts the first date of her last menstrual period (LMP). The child cannot be conceived prior to the woman ovulating, so identifying when she ovulated is key to understanding when she conceived. Since the date of ovulation/conception is calculated from the LMP, they just run with that.
It’s a little silly because when you are four weeks pregnant, the zygote/embryo has only existed for two weeks. It’s basically a historical artifact from a time when we knew very little about women’s fertility.
We have now studied female anatomy enough to know that this is only a very rough estimate. Women don’t necessarily ovulate on day 14. Women may have long or irregular cycles. If you live in a developed country, they will often revise the expected due date based on an ultrasound or on urine/blood testing if they look more than one week off. I use Natural Family Planning to track when I actually ovulate and my doctors/midwives have been willing to revise my due date based on that – so long as the other data fits.
Because you could have gotten pregnant at any time after that period ended. You’re technically more likely to get pregnant during ovulation but there have been cases of getting pregnant when the last time she had sex was in the middle of her period because the Soren survived long enough.
TLDR the last day of your last period is the last day you can *guarantee* that you weren’t pregnant.
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