Expert: Catherine Beier MS CBE - 6/19/2011

Question
okay so if i had a period and it may not have been a period it was implantation bleeding would a ultrasound date me from that bleeding or would it place me around the lmp i had before that bleeding? i know that questions seems confusing i just dont remember my lmp and i got an ultrasound and i really dont understand how they work in determinig pregnancy i had sex with 2 different guys in a period of like 1 or 2 weeks and im trying to figure out when the ultrasound gave me my weeks and days was that when i conceived because then it would be the first guys and not the second like i thought i had sex with one guy close to the end of the month then the other around the first week of the next month and what is a crl of 4.84cm mean?

Answer
Hello Tisha,


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 rate 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.


So, in your case, if you were given a CRL (crown-to-rump length) of 4.84cm, then you were most likely told you were approximately 11 weeks, 4 days pregnant at the time of the ultrasound.  Subtracting two weeks from this would give you the probable date of conception, or 9 weeks and 4 days before the ultrasound was performed.




Best,
Catherine Beier MS CBE
http://www.GivingBirthNaturally.com