QuestionI have asked you a question in the past and want to thank you again for your expert advice. I have read many of your past reponses to people about CRL dating. My questions are....
1. Where can I find a CRL dating chart? (I have found a few online but none of them match each other. I wanted to look at one that a medical professional would think is realiable.
2. Also how off with dating pregnancy can a first trimester ultrasound be? My ultrasound at 7w 3d was 12.2mm/12.6mm....how many days could this measurement be off in respect to dating?? (The second u/s was done at 11w 5d with a crl of 50.6mm.)
3. Are transvaginal u/s more accurate?
4. In the third trimester (28w 3d) is it true that the estimated weight using an ultrasound could be off by 1-2 pounds?
THANK YOU!
AnswerDear Michelle,
Although there are slightly different versions of CRL dating charts, most are pretty close. And here's the thing... different doctors and sonographers will use slightly different versions as well. Doctors acquire favorites based on their own personal experiences, etc,... Most are a "norm" and are not that far off from each other.
In general, ultrasonography dating of a pregnancy is more accurate than dating using the date of the mother's last menstrual period. The biometric CRL of the six to 12-week fetus provides the most accurate measurement - generally to within three days of true gestational age. From 12 to 18 weeks gestation, other anatomic structures are typically measured and biometric measurements are usually accurate to within a week of gestational age. After 26 weeks gestation, fetal variability has greatly increased and the dating provided by biometric measurement is less accurate, but the method is still used. By late in the third trimester, measurements are accurate only within a two-to three-week window.
In general, trans-vaginal sonography is superior to abdominal sonography in gestations less than or equal to 10 weeks, in obese patients, and in patients with retroverted uteri. The image clarity with T-V ultrasounds is almost always better, however T-V ultrasounds do limit views due to maneuverability issues of the probe.
If a mother is overweight or the fetus is in an awkward position, this can throw off the accuracy of the scan results in regard to measurements for fetal weight. However, at 28 3/7 weeks, the average weight of a fetus is just 2.22-2.54 lbs. The likelihood of a weight estimation being off by 1-2 lbs. is extremely low to impossible.
I hope this has helped you and answered your questions. I wish you well.
Brenda
