Assisted reproductive technology (ART) dating trumps all other sonographic dating. If the dating is off with ART, think about asking if the embryo was put in the uterus at 5 days, and not zero days, as that is how it is often calculated. This can be important if the embryo is larger than expected, as ART pregnancies have an increased incidence of Beckwith Weidman Syndrome, which is an overgrowth syndrome. If the embryo is smaller than expected, then the embryo should be followed more closely for possible congenital or chromosomal anomalies.

If the pregnancy is not ART, dating should be based on the 2014 ACOG/AIUM Committee Recommendations (Methods for Estimating the Due Date. Committee Opinion No, 700. American College of Obstetricians and Gynecologists. Obstet Gynecol 2017; 129:e150–154).

Measurement of the embryo or fetus in the first trimester is the most accurate method to determine gestational age. In the first trimester, the 2014 Recommendations state that if the pregnancy is less than 8 weeks 6 days, the embryo should be within 5 days of LMP (last menstrual period) and otherwise should be re-dated using ultrasound dates of the crown rump length. One mistake often made at this time is to include the gestational sac size in dating; the crown rump length is more accurate than the sac size and thus it should not be averaged into the estimated gestational age.

Crown rump length growth curves have been updated by Pexsters et al (Ultrasound Obstet Gynecol 2010; 35:650–655) using 4387 exams, whereas Hadlocks curves (Hadlock et al Radiology 1992; 182:501–505) were only based on 416 exams. These have some significant discrepancies in the 5–7 weeks gestational age range so we recommend using the Pexsters curves.

From 9 weeks to 16 weeks 6 days, the 2014 Recommendations suggest that the dating should be within 7 days and be re-dated if greater than “7-day” discrepancy.

From 16 weeks to 21 weeks 6 days, the 2014 Recommendations suggest that the dating should be within 10 days and be re-dated if greater than “10-day” discrepancy.

From 22 weeks to 27 weeks 6 days, the 2014 Recommendations suggest that the dating should be within 14 days and be re-dated if greater than “14-day” discrepancy.

From 28 weeks and beyond, the 2014 Recommendations suggest that the dating should be within 21 days and be re-dated if greater than “21-day” discrepancy.

We should not re-date a pregnancy in the second or third trimester if there are good ultrasound dates in the first trimester. If the patient gives “excellent dates” based on history (eg keeping a temperature chart, knowing date of conception based on specific dates of being with partner) and there is a greater than expected discrepancy of dates, then a follow-up sonogram should be recommended in 2–4 weeks, depending on the time of gestation (4 weeks in the second trimester and 2 weeks in the third trimester) so that appropriate growth can be assessed.

Serial growth is important in assessing dating. A fetus that grows 4 weeks in a 4-week period is likely dated appropriately. When the fetus grows more than 4 weeks in a 4-week period then accelerated growth should be reported, suggesting either an LGA (large for gestational age) or macrosomic fetus. History of prior pregnancies can be particularly helpful in these cases. Placement of the calipers at the outer edge of the subcutaneous tissue is particularly important in these cases. We often require 3 measurements, which are averaged to assess LGA/macrosomic fetuses.

When fetuses grow less than 4 weeks in a 4-week period then SGA (small for gestational age) or IUGR (intrauterine growth restriction) are suspected. Additional studies for well-being should be performed, such as umbilical artery Doppler, middle cerebral artery Doppler, maximum vertical pocket of amniotic fluid, biophysical profile, cerebroplacental ratio (CPR), or antenatal testing.

Do you have any tips on sonographic dating? Comment below or let us know on Twitter: @AIUM_Ultrasound.

Dolores H. Pretorius, MD, is a Professor of Radiology at University of California, San Diego, and Director of Imaging at UC San Diego Maternal-Fetal Care and Genetics, an AIUM-accredited practice.

Andrew D. Hull, MD, is a Professor of Clinical Reproductive Medicine at University of California, San Diego, and Director of UC San Diego Maternal-Fetal Care and Genetics, an AIUM-accredited practice.