To explore the normal reference range for fetal cardiac axis in the first trimester, and to evaluate its clinical value.

A total of 1639 unselected singleton pregnancies were enrolled from June 2017 to June 2018. Routine first trimester screening scan and fetal echocardiography were performed at 11^{+ 0}~13^{+ 6} weeks of gestation. The cardiac axis was assessed by obtaining an axial view of the fetal chest at the level of the four-chamber view (when the atrioventricular valve was completely closed).

The 1631 normal fetuses had a mean cardiac axis of (43.79±10.10)°, ranging from 10.80° to 74.02°. According to the gestational weeks, all pregnant women were divided into groups of 11^{+ 0}~11^{+ 6} weeks, 12^{+ 0}~12^{+ 6} weeks, and 13^{+ 0}~13^{+ 6} weeks, and the mean fetal cardiac axis for them were (43.40±10.82)°, (43.91±9.88)°, and (43.65±9.96)°, respectively; the difference among the three groups was not statistically significant (F=0.40375, P>0.05). The normal cardiac axis reference range was defined as 95% of the range of the fetal cardiac axis (23.99~63.58°). Among the 1639 fetuses, 8 were diagnosed with congenital heart disease in the first trimester, 6 of whom had abnormal cardiac axis value (4 cases of left axis deviation, 1 case of right axis deviation, and 1 case of unmeasurableness).

The reference range of fetal cardiac axis in the first trimester may be helpful in improving the detection rate of congenital heart defects, and it can be used as one of the indicators for the diagnosis of congenital heart defects in the first trimester.