Sonographic Evaluation of Fetal Growth in the Third Trimester of Low-risk Pregnancy: a Randomized Trial

The population to include in this study corresponds to the low-risk pregnant women with surveillance at the Prenatal Consultation or referenced by the Primary Care units to the Department of Obstetrics & Gynecology of Hospital de Santa Maria. The screening method for detection of fetuses at risk of fetal growth restriction currently used in low-risk pregnancies is the measurement of the symphysis-uterine distal distance at all visits from 24 weeks and a third trimester ultrasound at 30-32 (6 / 7 days weeks).

The patients who agree to participate in the study, after signing an informed consent, will be randomized into 2 groups (with and without an additional ultrasound evaluation at 35-35+6 (6 / 7 days weeks)). The randomization will be done through computer software.

Clinical data will be recorded such as: maternal age, ethnicity, parity, height, weight, socioeconomic status and smoking habits.

In the study group (ultrasound evaluation at 35-35+6 (6 / 7 days weeks)), the ultrasound evaluation will include biometric parameters of the fetus: cephalic perimeter, biparietal diameter, abdominal circumference and femur length. Based on these measurements, the computer system (Astraia) provides the estimated fetal weight and respective percentile according to the Hadlock formula. Amniotic fluid will also be measured (maximum column). Functional evaluation will include: Doppler of the umbilical artery, middle cerebral artery and Uterine arteries. The respective index of pulsatility and cerebroplacental ratio will be registered. All Doppler evaluations will be performed in the absence of somatic and respiratory fetal movements, under transitory maternal apnea, with the lowest insonation angle possible and at least 3 successive complexes will be evaluated for each measurement.

Obstetric and neonatal outcomes will be registered after delivery by revising medical records such as: gestational age at delivery, prepartum hemoglobin, type of labor (spontaneous or induced and respective indication), type of delivery (eutocic, instrumental, antepartum or intrapartum cesarean section), indication of instrumental or cesarean delivery, cardiotocographic register characteristics (repeated severe variable decelerations, late decelerations, absence of variability), evidence of meconial fluid, neonatal morbidity (hypoglycemia, hypothermia, transient tachypnea, hyperbilirubinemia requiring phototherapy, meconium aspiration syndrome, admission to neonatal intensive care unit, metabolic acidosis, intraventricular hemorrhage, necrotizing enterocolitis, sepsis, seizures and asphyxia) and mortality.

Sample size calculation

According to a retrospective analysis performed in the target population of this study, the rate of intrapartum fetal hypoxia requiring obstetric intervention in pregnant women with fetal growth restriction is 28%. Aiming to reduce this adverse perinatal outcome by 10%, it is estimated that the investigators will need to include a total sample of 2700 pregnant women (1350 in each group), with 80% power and α level of 0.05.

Performing third-trimester ultrasound at 30-32 weeks, the rate of antenatal detection of fetal growth restriction is 20.5% in low risk pregnancies. Aiming to increase the detection rate by at least 7%, the investigators will require a total sample of 1,200 pregnant women (600 in each group), with 80% power and α level of 0.05. Thus an interim analysis is expected to be performed when 1200 cases are recruited to the study to compare the detection rate of fetal growth restriction between the two groups.