QuestionHi Catherine,
Please let me begin by saying that I do understand that ultrasounds used for dating are most accurate in the first trimester. My question is; would maternal factors, as in smoking up to 10-11 weeks, increase the margin of error by any days? If so, what is the most "off" an ultrasound could be based on this? I did speak to a genetic counselor who assured me that it would not affect CRL but I did want to seek another opinion. Thank you in advance for taking the time to read this.
AnswerHello Rachel,
The genetic counselor was absolutely correct. The factors that influence the accuracy of the ultrasound are the quality of the equipment used, its timing, method, and the skill of the technician.
What you're really asking is whether maternal smoking up to 10-11 weeks of pregnancy can alter fetal growth rates, thus impacting the CRL obtained via ultrasound. But the answer to that is no as well.
Maternal lifestyle factors do not begin to affect a baby's growth rate until the placenta supports the pregnancy after the first 12 completed weeks of pregnancy. After that point, factors such as smoking, drug use, exercise and maternal nutrition can all affect a baby's growth rate and are why some babies are born full term at 5 pounds while others are 12 pounds or more.
During the first three months of pregnancy, cell division and multiplication is occurring which happens at a very consistent rate across all pregnancies, thus making it the optimal time to date a pregnancy.
Early ultrasound between 8-10 weeks is the most accurate window, with a standard error of measure of +/- 3 days at most, but typically +/- 1 day.
Smoking up to 10-11 weeks of pregnancy would not affect the baby's growth rate as the placenta is not yet responsible for sustaining the pregnancy and thus it would have no bearing on accuracy of an ultrasound.
Best,
Catherine
