Intrauterine Growth Restriction (IUGR)

iStock_000019664935XLarge-300x200When the estimated weight of your fetus is less than the 10th percentile it may be termed growth restricted (see the end of this post for a definition to help visualize the percentile measurement). Using the 10th percentile as a cutoff to define intrauterine growth restriction (IUGR) must be viewed as a screening tool to identify fetuses at risk for abnormal growth because 70 percent of fetuses with an estimated weight at less than the 10th percentile will be small due to normal explanations (such as small parents) and not pathological factors.

How is fetal growth monitored?

Your doctor, nurse practitioner, or midwife can assess the estimated fetal weight in a number of ways. One way is to measure from your pubic symphysis (the pubic bone) to the top of the uterus (called the fundus). That measurement in centimeters should equal your gestational age in weeks from 20 weeks to 34 weeks of gestation (obstetricians thank divine intervention for this time-growth symmetry). A second method called- Leopold’s maneuvers – named for the German obstetrician Christian Gerhard Leopold who developed this hands-on way of estimating fetal weight – involves feeling the uterus to estimate the fetal weight. Both measurement of the fundal height and Leopold’s maneuvers can be difficult to perform on obese patients and in the setting of excess amniotic fluid (termed hydraminos or polyhydraminos).

The most common way to gauge fetal growth is to use ultrasound measurements to derive an estimated fetal weight. Typically, the measurement of the head, abdomen and femur (thigh bone) are plugged into a complex formula programmed into the ultrasound machine to estimate the fetal weight. This estimate is then compared to published growth curves – to derive the growth percentile. Ultrasound measurements may also be used to establish individual growth curves for a fetus, similar to the way pediatricians monitor a child’s height and weight. Keep in mind that even ultrasound measurements are not particularly accurate.

What causes IUGR?

The causes of pathologic fetal growth restriction can be divided into: fetal, maternal, and placental explanations:

  • Fetal causes of intrauterine growth restriction include: aneuploidy (fetus has too many or too few chromosomes), anatomic malformations (most commonly cardiac), or fetal infection with microbes such as cytomegalovirus (CMV), toxoplasmosis, or other very rare infectious agents.
  • Maternal causes of fetal growth restriction include: maternal heart disease, renal disease, clotting disorders, lupus, infection, or toxic habits such as cigarette smoking, substance or alcohol abuse, maternal starvation, exposure to some medications, high altitude, or preeclampsia.
  • Placental causes of fetal growth restriction are probably the most common and include abruption (bleeding behind the placenta), clotting within the placenta, and structural abnormalities of the placenta.

Why is IUGR a problem?

Growth restriction increases the risk of stillbirth, which is devastating. Your doctor or midwife, and your maternal-fetal medicine consultant, will keep a watchful eye on your pregnancy to reduce the risk of stillbirth. Depending on the cause of IUGR, there may be other issues to be concerned about. Interestingly, IUGR is associated with obesity later in the fetus’ life.

The challenge is to distinguish abnormal IUGR from a fetus that is normal but was just destined to be small.

What will happen if my fetus is diagnosed with IUGR?

When a fetus has been identified as being at risk for pathological growth restriction by having an estimated weight at less than the 10th percentile, or has demonstrated decreased growth velocity on growth curves, your doctor or midwife may refer you to a maternal-fetal medicine specialist who will conduct a careful maternal history and fetal evaluation in order to establish a probable cause.

Then, you are likely to have regular fetal ultrasounds and fetal testing to monitor your fetus. Tests may include:

  • Ultrasound measurement of fetal growth (should not be done more than every two weeks).
  • Biophysical profile (an ultrasound test of fetal status)
  • Ultrasound evaluation of amniotic fluid volume
  • Doppler evaluation of blood flow through the umbilical artery
  • Non-stress testing.

What is the purpose of fetal testing if my fetus has IUGR?

The goal of testing is to identify a fetus with an abnormality that is at risk for stillbirth or another really bad outcome (who would benefit from early delivery) from the fetus that is just fine and would benefit from being left alone. Making this distinction may require multiple visits and many tests, and often relies on the experience of your doctor or midwife in consultation with a maternal-fetal medicine physician.

When should a fetus with IUGR be delivered?

We used to recommend delivery of all IUGR fetuses at 37 weeks, but recently the American College of Obstetricians and Gynecologists has recommended that fetuses with growth below the 10th percentile, but otherwise normal fetal testing, be delivered after 38-39 weeks.1 Abnormal fetal testing or ultrasound results may push your doctor or midwife to recommend earlier delivery.



Definition of percentiles in pregnancy:

Another way to think about the 10th percentile (or any other percentile) is to imagine lining up 100 fetuses that are exactly the same gestational age as your fetus from smallest to largest. The first fetus in line is the smallest, the second fetus is the second smallest, the 50th fetus in line is in the middle – the average estimated weight for that gestational age – all the way up until you get to the 100th fetus that is the largest. The 10th percentile is 10th in line, so nine fetuses are smaller and 90 are larger. In obstetrics, average is good, and we consider anything between 10 and 90 to be normal.

1.         ACOG Practice bulletin no. 134: fetal growth restriction. Obstetrics and gynecology 2013;121:1122-33.

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