First trimester ultrasound measurements, dating and guidelines…




Hey there, welcome back to another post of my series designated for ” PREGNANCY”, I wanna dedicate a blog for this topic because is very important, the CORRECT measurements in the first trimester are crucial to have an accurate due date, so here I will explain all you need to know about this.

So if you have the first scan and NO visible intrauterine pregnancy was seen but you have a Positive pregnancy urine test, the doctor will probably do a blood work to check levels of HCG in blood, in case you didn’t read my previous post I will link it below:

All you need to know about early gestation.

In the previous post I explained more in deep about HCG levels in the first trimester and when are you expected to see an intrauterine pregnancy, this topic of HCG is also important on the process of dating your pregnancy and give you an accurate due date.

So assuming that on your first scan only the Gestational SAC was seen which is normal at a 5 week ultrasound, then the gestational sac dimensions are taken and that will give us an idea of how far you are.



Ultrasound image of a gestational sac measuring 5 weeks 1 day.

However is not until we measure the Crown rump length ( CRL ) that the DUE DATE is set, here is why…

The most accurate sonographic measurement correlating with menstrual dates is the CRL, this measurement is the maximum visible length of the embryo, care must be taken not to include the umbilical cord or yolk sac in the CRL measurement.

Embryonic CRL measurement is more accurate than using Last menstrual period dates or physical exam in dating a pregnancy, remember if you suffer from Irregular periods due to PCOS or any other conditions then more likely Ultrasounds measurement are going to be different from your gestational age.


Ultrasound image of CRL measurement.


The left side of the table show length of the embryo in millimeters compared to the gestational age on the bottom.

The guidelines today for dating your pregnancy comparing CRL and menstrual age are listed on the table below, basically if there is a discrepancy of 5 days between the CRL and the gestational age and you are 8 weeks or less, the Due date will be set with the CRL Measurement and so on, take a look on the table…


Ok guys so this is the most important information about Dating your pregnancy , I want to thank you if you stay until the end , next post will be about Abnormal first trimester ultrasound , I will talk about Anembryonic pregnancy or Blighted ovum, Embryonic demise, and a quick review about Ectopic pregnancy and more, I will talk to you next time, if you are learning or enjoying my blog please give me a like and comment.

Remember as soon as we see an embryo we can measure a heartbeat, normal ranges are from 115 to 160 beats per minute, varies thought out the pregnancy, it increases at around 10 weeks to 150 to 175 beats per minute and decreases again approaching to term to 130 beats per minute.

xo zadi.

And remember you got this …. GOOD LUCK!!!!

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Hi, welcome to my site, I am Zadi, ultrasound tech and mom of 2 boys. Being a mother is one of the most beautiful things that have happened to me, but motherhood can be frustrating and difficult sometimes. I am sure any mom can relate to this. With this blog, I want to help you navigate the stages of pregnancy and parenting gracefully, by sharing my medical knowledge and my experience as a mom with you.

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