10/2024
What is fetal growth restriction (FGR)?
A fetus is determined to have growth restriction when the overall estimated fetal weight (EFW) is <10% and/or the abdominal circumference (AC) is <10% for gestational age (GA). It is very important that the pregnancy is accurately dated-the best due date has been determined-before diagnosing FGR. FGR occurs in up to 10% of pregnancies and is a leading cause of infant morbidity and mortality. It can result from a variety of maternal, fetal, and placental conditions:
- Chromosomal disorders and congenital malformations are responsible for approx. 20% of FGR cases.
- Suboptimal perfusion of the maternal placental circulation is the most common cause of FGR and accounts for 25–30% of all cases.
Severe FGR is diagnosed when the EFW <3% for GA and early-onset FGR is diagnosed when it occurs at <32 weeks of pregnancy.
Screening for FGR in pregnancy
Starting in the the second half of pregnancy, your obstetrical care provider will check how your fetus is growing during your prenatal care visits by measuring the fundal height (FH). This is done by measuring your abdomen from the pubic bone to the top of the uterus (the fundus). Fundal height (FH) measurements in centimeters (between 24–38 weeks) at prenatal visits approximates the GA and is used to screen for FGR in every pregnancy. If the FH is 3 or more centimeters less than the GA of the pregnancy, that should trigger assessment with a fetal growth ultrasound. FGR can also be picked up if growth ultrasounds are being done due to medical complications of pregnancy or if there are risk factors present (see below). Fundal height measurement may not work as well in people who are obese or who are pregnant with more than 1 baby.
What are some risk factors for FGR?
- Maternal Medical Disease
- Hypertensive disorders
- Kidney disease
- Pregestational diabetes mellitus
- SLE
- Antiphospholipid syndrome
- Cyanotic heart disease, Reduced cardiac output
- Placental Anomalies
- Abruption
- Infarction
- Circumvallate placental shape
- Umbilical cord anomalies
- Velamentous or marginal insertion
- Single umbilical artery
- Fetal Factors
- Multiple gestation
- Medications
- e.g. fetal hydantoin syndrome)
- Substance use and substance use disorder
- Alcohol
- Cocaine
- Smoking
- Intrauterine infections
- CMV
- Rubella
- Syphilis
- Aneuploidy (chromosomal/genetic differences)
- Trisomies 13 and 18
- Congenital malformations
- Cardiac abnormalities
*All of these things can result in suboptimal uterine–placental perfusion and fetal nutrition.
What will happen once FGR is diagnosed?
If you receive a diagnosis of FGR in your pregnancy at <32 weeks of pregnancy, you may be referred to a genetic counselor. FGR occurring at <32 weeks is associated with fetal chromosomal differences and genetic syndromes. As a result having prenatal genetic screening testing and genetics counseling is important. If there is FGR AND an abnormality on the fetal anatomy, the risk of the fetus having a chromosomal (genetic) difference is increased. A detailed fetal anatomy ultrasound with a fetal echo should also be done if not done already. A diagnostic amniocentesis for a chromosomal microarray analysis may be offered if there is a high suspicion for a genetic abnormality in the fetus. This diagnostic testing may also be recommended when FGR is present and a fetal malformation/defect, polyhydramnios (extra amniotic fluid), or both are also present regardless of GA.
You may be referred to a Maternal-Fetal Medicine (MFM) specialist for consultation and co-management of your care. In some cases, care may be transferred to the MFM. Ask your ObGyn.
Serial fetal growth and amniotic fluid ultrasounds every 3-4 weeks will be recommended, in addition to evaluating the blood flow from the placenta to the fetus in the umbilical cord with weekly umbilical artery (UA) doppler studies. The UA doppler studies will start when the FGR is diagnosed or at 24 weeks of pregnancy, whichever comes first.
Fetal surveillance with biophysical profiles (BPP) and/or non-stress tests (NST) will be recommended. BPPs typically start at 26 weeks and can be done weekly with the UA doppler studies. At around 32 weeks of pregnancy the BPPs can be stopped and twice weekly NSTs started. Each center may have a different version of how they do fetal surveillance when FGR is diagnosed.
Screening for toxoplasmosis, rubella, or herpes in pregnancies with FGR in the absence of other risk factors for these infections is not recommended.
There are no proven treatments for a diagnosis of FGR. There is insufficient evidence to routinely administer aspirin, lovenox or heparin to treat FGR in a current pregnancy. Bed rest is NOT recommended for a diagnosis of FGR cause harm by increasing your risk of bone loss, muscle loss, and blood clots.
Perinatal complications of FGR
FGR is associated with a significantly increased risk of stillbirth, with the most severely growth restricted fetuses being at greatest risk. This risk is also increased if there is a co-existing genetic or fetal structural abnormality. At fetal weights less than the 10th% for GA, the risk of fetal death is approximately 1.5%, which is twice the background rate of fetuses of normal growth. The risk of fetal death increases to 2.5% at fetal weights less than the 5th% for GA. If the UA doppler studies and significantly abnormal, ie absent or reversed end-diastolic flow, these fetuses are at particular increased risk of adverse outcomes and have an increased frequency of neonatal mortality and morbidity. If your pregnancy is diagnosed with FGR, it is important to ask where you will deliver and if NICU care is available.
Once the growth restricted fetus is delivered, It is likely they will need to go to the NICU for care. These neonates are predisposed to complications, including hypoglycemia, hyperbilirubinemia, hypothermia, intraventricular hemorrhage, necrotizing enterocolitis, seizures, sepsis, respiratory distress syndrome, and neonatal death. Again, this depends on the severity of the growth restriction, GA when delivery occurred and if there are co-existing genetic or fetal structural abnormalities. FGR has also been linked to an increased risk of certain health issues later in life, such as heart disease and diabetes, learning disabilities and problems with behavior. However, not all pregnancies with FGR will have these problems.
When and how will delivery occur?
When to deliver will depend on the severity of the FGR, the UA doppler studies and the GA of the fetus. The goal is to get the pregnancy as far along as possible as long as the benefits of staying pregnant outweigh the risks of delivery. There are recommendations available for timing of delivery. If an early delivery is needed based on the severity of the FGR and/or UA doppler studies, antenatal corticosteroids for fetal lung maturity and/or magnesium sulfate for fetal neuroprotection may be recommended prior to delivery.
Cesarean delivery should only occur if there are other maternal and/or fetal indications and not solely because of a diagnosis of FGR.
What does this mean for future pregnancies?
There is a 20% recurrence risk for FGR in a subsequent pregnancy. If there were any modifiable factors or maternal medical disease present in the FGR-affected pregnancy, it is important that these be addressed and optimally managed prior to the next pregnancy. There are no proven treatments of strategies to prevent FGR from occurring in a subsequent pregnancy. There is insufficient evidence to routinely administer aspirin, lovenox or heparin to prevent FGR in a future pregnancy. There is no evidence that nutritional and dietary supplemental strategies for the prevention of FGR are effective.
Resources
ObG Project: Fetal Growth Restriction: Definition, Evaluation and Management
Fetal Growth Restriction Before and After Birth
SMFM Recommendations: FGR Diagnosis and Management