Understanding Ultrasound Dating During Early Pregnancy

Question
QUESTION: okay you have answered a couple questions for me i really appreciate it but i have one last question how come when i got an ultrasound it placed me around the time i think i had a period after intercourse if i didnt member my lmp that is what is confusing about all this because it placed me back at the end of octber instead of in november when i started having sex with the new guy but November is also the month i missed my period im just confused on how an ultrasound can date you off lmp if you dont remember it or does it date you off conception i get so many different answers???? but this is my last question i promise thank you so much

ANSWER: Hello Jakira,

Ultrasound does not date you off your last menstrual period.

It can be helpful to know the date of your last period, but it is not mandatory. Ultrasound looks at different fetal markers, including crown-to-rump length (CRL), diameter of the gestational sac, and in later ultrasounds, femur length, abdominal circumference and other such factors.

In an early first trimester ultrasound, the CRL and gestational sac size are determined to date the pregnancy.  In the first weeks of pregnancy, fetuses grow at much the same rates as cells divide and multiply at very similar rates.  It is only during the late first trimester/early second trimester when the placenta takes over to sustain the pregnancy that individual growth rates come into play determined by genetics, diet, maternal health, lifestyle, etc.

So, an early ultrasound will measure fetal markers such as those above and then yield dates based on those measurements to determine the fetal age, which is different from the gestational age.  The gestational age is approximately two weeks more than fetal age and should roughly coincide with the first day of your last period.  Conception typically occurs about 14 days before your next period is to begin, which is when the fetus would start to form and is why fetal age is 2 weeks less than gestational age.


Since pregnancy is measured by the gestational age (which includes approximately 2 weeks where you are not pregnant yet, as it's determined by adding two weeks to the fetal age which for most women with regular cycles will be in line with the first day of their last period), after your ultrasound, you will be told the gestational age or how many weeks pregnant you are. Subtracting two weeks will tell you approximately when the baby was conceived.


This is why you would be told the end of October, not November, because it should be about two weeks before you actually conceived.  





Best,
Catherine

---------- FOLLOW-UP ----------

QUESTION: Okay thank you so much you really made it clear so basically when they tell you how far along after your ultrasound or on a ultrasound scan is from your lmp and not conception? i was just wondering if i should have told the october guy if it was a possiblity it could be his because i didnt remember my lmp

Answer
Jakira,

An ultrasound measures the fetal age and then the results are given to you in terms of gestational age (adding two weeks as if from your LMP) as that's how pregnancy weeks are calculated. Pregnancy is measured as 40 weeks from your LMP (gestational age), 38 from conception (fetal age).


Best,
Catherine