Fetal growth restriction: Clinical sciences

Fetal growth restriction, or FGR, is also known as intrauterine growth restriction, or IUGR; and is defined as a fetal weight or abdominal circumference below the 10th percentile for gestational age, as determined by ultrasound measurements. In contrast, small for gestational age, or SGA, refers to a term newborn with a birth weight less than the 10th percentile for gestational age. FGR can result from a variety of maternal, placental and fetal conditions. It requires increased antenatal surveillance, possibly early delivery, and it contributes to poor perinatal outcomes and negative long-term health impacts for the child.

When a patient presents with a chief concern suggesting fetal growth restriction, your first step is to perform a focused history and physical examination. Key findings on history include prior delivery of an SGA infant, multiple gestation in the current pregnancy, and inadequate maternal weight gain with or without a maternal malabsorptive syndrome. Other risk factors include an intrauterine infection like cytomegalovirus, toxoplasmosis, rubella, or varicella zoster; maternal chronic disorders with a vascular component such as chronic hypertension, long-standing, poorly controlled diabetes, or chronic kidney disease; or a history of antiphospholipid syndrome, also known as APS

Here’s a clinical pearl! Acquired thrombophilias such as APS can contribute to FGR, but inherited thrombophilias do not.

Finally, be sure to ask about any substance use during pregnancy, like tobacco or alcohol, as well as teratogen exposure, including medications like valproic acid, cyclophosphamide, and warfarin.

On physical examination, note your patient’s weight and ensure adequate weight gain. Beginning at 24 weeks, measure the fundal height and note whether it is appropriate for the gestational age, by measuring from the top of the pubic symphysis to the fundus of the uterus using a flexible measuring tape.

The measurement should approximate the gestational age in weeks, plus or minus 3 centimeters. While fundal height is a useful screening tool for FGR, keep in mind that obtaining an accurate measurement can be difficult in the presence of adiposity or large uterine fibroids.

If there’s a discrepancy of more than 3 centimeters between the expected fundal height and weeks of gestational age, this is typically referred to as “size less than dates” and may indicate the fetus is growth restricted. In this case, your next step is to perform a growth ultrasound to evaluate the estimated fetal weight, or EFW, and amniotic fluid volume.

Here’s a clinical pearl! For pregnancies with identifiable risk factors for FGR, a growth ultrasound is routinely recommended as early as 28 weeks, regardless of the fundal height measurement.

Next, check the criteria used to date the pregnancy to ensure that the estimated due date, or EDD, utilized for the calculation of EFW is appropriate. The most reliable method for determining the EDD is a first-trimester ultrasonographic crown-rump length. Alternatively, a pregnancy is considered to be suboptimally dated if no ultrasound examination, confirming or revising the EDD, is performed prior to 22 weeks of gestation.

Okay, once you have confirmed the dating, review the results of your growth ultrasound. The EFW is determined by measuring four main parameters: the biparietal diameter, head circumference, abdominal circumference, or AC, and femur length.

If the EFW or the AC is less than the 10th percentile for gestational age, you can diagnose FGR.

Once confirmed, perform a detailed obstetrical ultrasound including fetal anatomy, if not previously completed, and look for any fetal anomalies. Also, assess and document the gestational age at which FGR is diagnosed.

Now, if your patient is diagnosed with FGR before 32 weeks of gestation, it is typically referred to as early onset FGR.

Early onset FGR is usually attributable to fetal causes including genetic, structural, or infectious causes. As such, all patients with early onset FGR require referral for genetic counseling and should be offered fetal diagnostic testing, including chromosomal microarray analysis, or CMA.

This is performed via chorionic villus sampling or amniocentesis, with amniocentesis being the test of choice after 15 weeks of gestation. Additionally, be sure to review the findings of the detailed ultrasound to identify any structural causes for FGR, such as congenital heart disease, diaphragmatic hernia, abdominal wall defects, or neural tube defects.