We have known for some time that infants who are growth restricted are more prone to problems related to oxygen deprivation and have a higher chance of dying in utero, dying during labor and delivery, and dying during the first hours, weeks, and months of life.
We have learned more recently, moreover, that fetal growth restriction has long-term adverse consequences that extend into adult life. Epidemiologic studies in England in particular show that infants who were growth restricted in utero have a higher chance of developing diabetes, hypertension, stroke, and cardiovascular disease. Significant attention has been paid to the Barker hypothesis, which theorizes that the cardiovascular and endocrine systems of growth-restricted fetuses undergo a sort of intrauterine programming caused by a compromising prenatal environment.
Many aspects of the causes and pathophysiology of growth restriction remain unclear, and none of the therapeutic approaches that have been tried to improve fetal condition—from maternal oxygen administration, various nutritional interventions, and pharmacologic agents to plasma volume expansion and abdominal compression—have been consistently successful or valuable.
However, we have made advances in our understanding of the mechanisms and perinatal risks. We also have made significant progress in diagnosis and management and can today follow an evidence-based approach for managing the complications.
We are at the point today where our role as obstetricians can and should be to identify patients at risk of fetal growth restriction, to sonographically diagnose fetal growth restriction in at-risk patients, to monitor growth-restricted fetuses for in utero compromise, and to ensure a timing of delivery that will maximize gestation while minimizing the risks of continuing the pregnancy.
Identifying the Problem
Fetal growth restriction—or intrauterine growth restriction, as it is sometimes called—refers to the failure of the fetus to realize its optimal growth potential. A baby should be considered growth restricted when sonographically measured fetal dimensions—particularly the abdominal circumference or the estimated fetal weight based on head circumference or diameter, abdominal circumference, and femoral length—deviate below the 10th percentile for gestational age.
Some have advocated for a more rigorous threshold of the 5th percentile, or even the 3rd percentile, and others have suggested using the 15th percentile as a cutoff. The 10th percentile is indeed arbitrary, but for now it is the most commonly used threshold and should be considered the current standard of practice. It strikes the right balance.
Since subnormal growth is defined using gestational age-specific standards, we must establish gestational age as early in the pregnancy as possible, preferably in the first trimester. We must also be as accurate as possible, since overestimating or underestimating the gestational age by even a few days can have significant clinical implications for the discovery of fetal growth restriction.
Use of the crown-rump length presents us with a possible 4- to 5-day variation in gestational age, which is significant but still better than a 2-week variation.
Menstrual history in general is not very reliable, but if there is good concordance between gestational age based on menstrual history and that based on crown-rump length, then one can use the menstrual age. If there is more than a 1-week difference, then I advise using the crown-rump length.
One of the major issues we face in dealing with fetal growth restriction is, of course, that not all babies who are small are abnormal; some are just constitutionally small. Similarly, some babies appear to be normal—and may even be of an appropriate weight for their gestational age—but in reality are facing uteroplacental insufficiency and are not realizing their growth potential. There is, therefore, a definite “gray area” in distinguishing those babies who are truly growth restricted.
This is one reason why the best diagnostic test for fetal growth restriction is serial ultrasonography. Once we identify patients at risk for fetal growth restriction—from those who have preeclampsia and other hypertensive conditions to those with perinatal infection and smoking or substance-abuse problems—we can follow that patient and fetus to get a better sense, for instance, of whether the fetus is small but growing normally or small with progressively declining growth. We can also apply the 10th percentile threshold.
In general, routine growth scans in at-risk patients should start at around 28 weeks and be done every 3–4 weeks unless a diagnosis of growth restriction is made. If a problem such as preeclampsia becomes evident at another time, then serial ultrasonography should commence.
Recognizing the Risks
There is a host of disorders—various maternal, fetal, and placental factors—that can interfere with the mechanisms that regulate fetal growth.