The Value of an Ultrasound at the First Prenatal Visit

Abstract & Commentary

Synopsis: A routine ultrasound assessment for dating at the first antenatal visit allows a more accurate estimation of gestational age and decreases the need to adjust gestational dates later in pregnancy. In addition, women who have an ultrasound at their first prenatal visit have more positive feelings about their pregnancy.

Source: Crowther CA, et al. Br J Obstet Gynaecol 1999;106:1273-1279.

To determine the value of an ultrasound scan at the first prenatal visit, Crowther and colleagues performed a prospective, randomized clinical trial of 648 women presenting at less than 17 weeks gestation. Women who had no previous ultrasound and no indication for a scan were randomized to have an ultrasound performed at that time or to a control group. Patients presented at 10-11 weeks gestation. The main outcome measures included the number of women who needed adjustment in their dates by 10 days or more when an ultrasound was performed to assess fetal anatomy at 18-20 weeks, as well as the number of women who were scheduled for their fetal anatomy scan or who had maternal serum screening tests at inappropriate gestational ages. Women in both study groups also completed a questionnaire at the first prenatal visit to assess their feelings about the pregnancy.

For those women randomized to receive an ultrasound at the first prenatal visit, 24% required a correction in gestational dating. Among women who said they were certain of the date of their last menstrual period, 18% needed adjustment of their dates. Not surprisingly, significantly fewer women who had an ultrasound at their first visit needed an adjustment of their due date when an ultrasound was performed at 18-20 weeks (9% vs 18%). In the control group, no increase was observed in the number of women who were scheduled inappropriately for their 18- to 20-week scan or the number of repeat maternal serum screening tests required due to incorrect dating. Of note, 25% of women in the control group did have a scan before 18-20 weeks, most often for vaginal bleeding or uncertain gestational age. No significant differences were noted in the number of nonviable pregnancies detected, the number of twins diagnosed at an earlier gestational age, or the number of major anomalies detected. Women who had an ultrasound at the first visit were less worried about their pregnancy.

Crowther et al conclude that a routine ultrasound assessment for dating at the first antenatal visit allows a more accurate estimation of gestational age and decreases the need to adjust gestational dates later in pregnancy. In addition, women who have an ultrasound at their first prenatal visit have more positive feelings about their pregnancy.

Comment by Steven G. Gabbe, md

Many obstetricians perform an ultrasound at the first prenatal visit to establish gestational dating. How helpful is this practice in a general obstetric population? In this prospective, randomized trial, an ultrasound performed at the first obstetric visit, most often at 10-11 weeks gestation, provided more accurate dating than the patient’s menstrual history in a significant number of women and reduced maternal anxiety. Crowther et al did not examine the cost-effectiveness of this approach. Their findings are supported by a retrospective study by Barrett and Brinson, who examined the value of an ultrasound at the first prenatal visit in 1000 consecutive patients in Barrett’s private practice.1 Nearly three-fourths of the patients were in the first trimester. Important information was obtained in more than one-third of the patients: a significant dating discrepancy,26.8%; missed abortion ,5.7%; twins,1.6%; uterine fibroids, 1.2%; and fetal anomalies, 0.5%. When compared to other studies routinely performed at the first prenatal visit such as a urine culture or a serologic test for syphilis, Barrett et al conclude that the ultrasound was more likely to yield helpful information.

Recognizing that the cost-effectiveness of this practice has not been clearly demonstrated, it is my opinion that for many obstetric patients, especially those with uncertain menstrual dates and those who are highly likely to need an elective delivery, an ultrasound examination performed at the first prenatal visit in the late first or early second trimester is valuable.

Reference

1. Barrett JM, Brinson J. Am J Obstet Gynecol 1991;165: 1002-1005.

In the prospective, randomized clinical trial performed by Crowther et al, an ultrasound performed at the first prenatal visit significantly improved which of the following?

a. Detection of twins

b. Detection of major fetal malformations

c. Recognition of a nonviable pregnancy

d. Accurate gestational dating

e. Placental localization