To investigate the cardiac axis changes within the cardiac cycle of both normal fetuses and fetuses with congenital heart diseases (CHD), and construct reference range of cardiac axis for normal fetuses.

Two hundred sixty-five fetuses were enrolled in our study, of whom 197 were normal fetuses and 68 were fetuses with CHD. The cardiac axis was measured at end systole (CAS°) and at end diastole (CAD°) in each case. Subsequently, all parameters were compared between the normal and CHD groups. The independent t-test was used to compare CAS° and CAD° parameters between the healthy control group and the middle pregnancy group and the late pregnancy group, and between the healthy control group and the CHD group. According to the practice guideline the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG): the normal reference range for normal fetal cardiac axis was (45±20)°, with<25° and/or>65° defined as abnormal fetal cardiac axis. The Chi-square test was used to compare the proportion of CAS° and CAD° CA abnormalities between the healthy control group and the CHD group.

No significant difference was observed in comparison of maternal age and gestational age between 197 healthy controls and 68 CHD controls (P>0.05). There was no significant difference between CAS° and CAD° of the fetus in the healthy control group with the increase of the second and third gestational weeks (P>0.05). The mean cardiac axis in control group was (40.88±10.27)° at end systole, and (35.07±9.44)° at end diastole, resulting in an average difference of (5.85±4.42)°. While the mean cardiac axis in fetuses with CHD was (44.34±15.03)° at end systole and (41.32±16.71)° at end diastole, resulting in an average difference of (5.01±3.52)°. Significant difference was observed between two groups (t=-2.11,-3.79, all P<0.05). Among 197 normal cases, there were 11 cases with abnormal CAS° (4 cases with decreased CAS° and 7 cases with increased CAS°) accounting for 5.6%. There were 27 cases with abnormal CAD° (26 cases with decreased CAD° and 1 case with increased CAD°), accounting for 13.7%. In 68 CHD cases, there were 10 cases with abnormal CAS° (4 cases with decreased CAS° and 6 cases with increased CAS°) accounting for 14.7%. There were 17 cases with abnormal CAD° (12 cases with decreased CAD° and 5 cases with increased CAD°) accounting for 25.0%. The abnormity rates of both CAS° and CAD° were significantly lower in normal group than in the CHD group. The difference was statistically significant (χ²=5.765, 4.657, all P<0.05).

Measurement of cardiac axis at end systole and at end diastole provides values in screening for fetuses with congenital heart disease, and can be used as a routine measurement tool.