Fetal echocardiography carried out at
weeks on fetuses at risk of CHD allowed distinction between normal and abnormal cardiac appearances in the majority of cases (96%). There was a relatively high number of cardiac abnormalities (9%) but early reassurance was possible for most families. For those with an abnormal early scan, the identification of cardiac defects, in isolation or associated with other fetal abnormalities, contributed to a very high rate of termination of pregnancy (76%). Among 199 fetuses considered to have a normal heart at the time of the first examination, 29 (15%) were subsequently shown to have an adverse outcome related to non-cardiac causes, reflecting the high risk of the population studied.
Most CHD encountered in our study were seen in fetuses who had increased NT thickness (16 of 21 cases). Measurement of NT in the first trimester is a non-invasive way of assessing risks of chromosomal abnormalities.8
It is well established in a number of centres in the UK9
and elsewhere. For chromosomally normal fetuses with increased NT, the risk of CHD increases with NT thickness.4,10–12
Mavrides and colleagues11
have shown that in an unselected population, the risk for major CHD increases from 3.1 cases in 1000 (normal NT thickness < 2.5 mm) to 50 abnormalities in 1000 (5% risk) if NT is above the 99th centile for crown to rump length (NT
3.5 mm). The latter is similar to the study of Ghi and colleagues,12
who reported a 7% incidence of major CHD in chromosomally normal fetuses with NT > 3.5 mm.
When the criteria for a normal cardiac scan were met, families could be reassured early during their pregnancy, This, together with extracardiac fetal assessment and exclusion of other major fetal abnormalities, was important for continuation of pregnancy. Follow up data showed no major cardiac defect in all but two fetuses, one of whom had a progressive lesion requiring neonatal intervention and the other a large VSD. The former case confirmed observations made by others regarding the progression of certain lesions during pregnancy13
but accounted for < 1% of all normal early scans. This case does, however, stress the need for later follow up both to ensure continuing adequate growth of all cardiac structures and to confirm the early ultrasound findings of normality. Mid-second trimester scans would provide better definition of the septal structures and ascertainment of normal pulmonary venous connections.
That a complex structural malformation of the heart can be diagnosed accurately in early pregnancy has been confirmed in all cases where the heart was available for postmortem examination. The identification of such defects inevitably contributes to a family’s decision to interrupt the pregnancy wherever this is a legal option. Particularly if there are no other major fetal abnormalities, the need for accuracy in the cardiac diagnosis is imperative. To confirm this, as well as to counsel families adequately for subsequent pregnancies, a request for a postmortem examination should be mandatory. High quality pathological assessment of such small hearts, however, is not widely available. This poses difficulties that will need to be addressed by those undertaking early fetal echocardiography.
A group of fetuses presented with disproportion between the right and left sided structures, the natural history of which is still unknown.14
In this group, the early scan raised the possibility of CHD, which may develop further as pregnancy progresses, yet it provided neither the reassurance of a normal scan nor sufficient information to allow families to decide to interrupt the pregnancy because of a fetal cardiac abnormality alone. In our series, five of seven fetuses that presented with asymmetry between right and left sided structures had greatly increased NT, an associated chromosomal abnormality, or both. Two had diaphragmatic hernia, one with increased NT and one with asymmetry but a normal karyotype and normal NT measurement. In one case, later scans were indicative of coarctation of the aorta. When there is ventricular disproportion, the early scan may or may not allow sufficient delineation of the aortic arch to show associated narrowing of the aortic isthmus. This may only be documented with postmortem examination. Hyett and colleagues3
showed that increased NT and isthmal hypoplasia are associated in early pregnancy but, in the absence of other cardiac abnormalities, it is not possible to predict what the postnatal findings will be if a pregnancy continues.
Imaging the fetal heart in the late first and early second trimester of pregnancy is technically more demanding than in mid-gestation because of the relatively smaller size of the fetus and cardiac structures (the semilunar valves are about 1.2 mm at 13 weeks). The quality of images also depends on maternal size and fetal position. The use of colour flow mapping and power Doppler provides useful additional information that allows pattern recognition of the cardiac connections and valve function. The value of this additional information cannot be underestimated.
Our study confirms the feasibility and utility of early cardiac scans for high risk pregnancies, particularly regarding early reassurance. It stresses the need for a combined fetal medicine approach to include cardiac and extracardiac fetal assessment, particularly if NT is increased, whether or not the fetal karyotype is normal.