In the majority of patients, up to 12 parameters of fetal cardiac structure can be visualized. Each of the three segments of the exam takes only a few seconds to perform, so the actual collection of information is rapid. The technologic advances have also made the acquisition of images easier and less operator dependent. Moreover, the analysis is then performed offline, so the mother can go home afterward. Offline analysis of images also means that the ultrasound scan itself can be performed by trained sonographers at a distance from a cardiac center, with the information transmitted to the center for expert analysis.
It wasn't long ago that second-trimester fetal echocardiography was the gold standard for any prenatal evaluation of fetal cardiac structure and function. Now, with an early and integrated screening approach that utilizes first-trimester fetal cardiac examination, we can in fact diagnose many of the most severe heart defects as early as 12 weeks of gestation. At this stage, the fetal heart is as small as the tip of the little finger.
This component of first-trimester screening is just now coming to the forefront. Its availability can benefit populations at high risk of cardiac anomalies (such as women who have long-standing diabetes). It may be especially beneficial to those who were in poor glycemic control at the beginning of their pregnancy. It appears, though, that the exam can be meaningfully applied in low-risk populations as well. Research is underway to determine the best approaches to counseling and to determine which patients should have subsequent invasive testing.
Other New Frontiers
Another area of interest is the potential ability to predict which women will develop preeclampsia later in pregnancy based on how the fetus and placenta are faring at approximately 12 weeks' gestation.
Doppler investigations have shown us that placental abnormalities are difficult to distinguish from normal placental development early in pregnancy. In the first trimester, therefore, Doppler alone is a fair mechanism for knowing whether placental development is deficient enough to put the mother at high risk for developing preeclampsia or isolated hypertension.
However, when Doppler is combined with measurement of a family of maternal serum analytes – some of them inflammatory substances and some of them chemicals that regulate the formation of blood vessels – it can be employed to predict who will develop early hypertensive complications. And when other factors such as maternal weight and blood pressure at the time of first-trimester assessment are added to the equation, the accuracy of our predictions increases further.
We are proceeding in this area with a bit of caution, as we cannot yet predict the onset of preeclampsia later on in pregnancy. The predictive value of the first-trimester assessment for hypertensive problems that occur closer to term is not very good, so patients with normal early assessments still need careful prenatal care.
Still, in many ways we can tackle the most severe problems through early detection. There is some evidence that the administration of low-dose aspirin can reduce the incidence of hypertension and preeclampsia, as well as complications with the baby's growth, in women with detected placental abnormalities. This means that not only are we able to define and identify those women at highest risk, but we also have the ability to potentially modify the course of placental development and perhaps even eliminate hypertensive complications.
Current research is aimed at defining who will best benefit from this approach, because while low-dose aspirin appears in some research to work when started early in high-risk women, benefits have not been duplicated in other studies.
More broadly, first-trimester assessment of maternal characteristics (such as weight), serum analytes, and ultrasound features set the stage for ongoing maternal evaluation of characteristics such as weight gain during pregnancy to predict her risk of developing preeclampsia, diabetes, and other serious problems, including neonatal concerns requiring specialized newborn care.
The Big Picture
As first-trimester screening evolves with technologic developments to become more comprehensive and precise, one of its ever-important components involves the art of history taking, physician-patient dialogue, and the incorporation of low-tech risk assessments for coping with and possibly preventing preterm labor and delivery.
Measuring the cervix at this very early stage is not a good predictor of its ability to contain the pregnancy for the rest of the gestation or even until a reasonably mature gestation is reached. In the first trimester, the cervix generally is not under enough pressure from the weight of the pregnancy to disclose whether it is a strong or weak cervix or whether it has the potential to shorten in an extreme way or not. This is different from measuring the cervix later in pregnancy when the shortening process has already started, and when intervention is based on proven results.