Chapter 33: Labor in the Presence of Fetal Complications

FETAL GROWTH RESTRICTION

Background

Fetal growth restriction (FGR) occurs when the fetus is unable to achieve its full in utero growth potential, leading to increased risk for significant morbidity and mortality. This can be due to genetic or congenital malformations, placental conditions, congenital infections, and maternal medical conditions.

Maternal placental hypoperfusion accounts for 25 to 30 percent of all cases of FGR, while chromosomal disorders and congenital malformations are responsible for approximately 20 percent.

Pregnancies complicated by FGR have increased risk for adverse perinatal outcome including but not limited to preterm birth, stillbirth, neonatal morbidity, and mortality. Furthermore, those fetuses are at risk of long-term disabilities such as neurological (cognitive and learning disabilities), cardiovascular, and endocrine diseases.

FGR is classified as “early onset” when it occurs before 32 weeks and as “late onset” after 32 weeks’ gestation.

Small for gestational age (SGA) is a terminology often misused to describe fetal growth restriction. It actually refers to neonatal birth weights below the 10th percentile, while FGR relates to the estimated fetal weight (EFW).

It is important to highlight that approximately 18 to 22 percent of FGR cases are healthy fetuses and considered constitutionally small at birth with a normal outcome.

Diagnostic Criteria

Differentiation between constitutional and pathological FGR remains a prenatal challenge for health care providers. There is a lack of agreement and broad consensus in the diagnostic criteria among different international guidelines from Canada, the United States, the United Kingdom, and International Societies such as the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG).

Commonly, FGR would be defined as an EFW below the 10th percentile. This diagnostic criterion has high sensitivity but low specificity, including babies who are constitutionally SGA, but healthy and with normal perinatal outcomes.

The inclusion of other parameters such as abdominal circumference (AC) and Doppler improves the specificity of the diagnosis.

The Society of Obstetrics & Gynecology of Canada (SOGC) and The Society for Maternal Fetal Medicine in the United States (SMFM) both define FGR with an EFW or AC below the 10th percentile.

ISUOG Guidelines for defining FGR are based on the International Delphi Consensus using EFW and/or AC and may also include the fetal growth curve and maternal and fetal Doppler studies.

Differentiating between constitutional and pathological FGR has important impacts on antenatal surveillance, timing of delivery, and labor and delivery management (Fig. 33-1).