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The ability to differentiate fetuses either small for gestational age (SGA) or with intrauterine growth restriction (IUGR) is limited. Sonographic estimated fetal weight (EFW) is based on abdominal circumference (AC) or an EFW measurement less than the 10th centile. This study was undertaken to evaluate which sonographic parameters are associated with perinatal morbidity and mortality in pregnancies affected by IUGR to establish definitive criteria for diagnosis of IUGR.The PORTO study was conducted at 7 academic obstetric centers in Ireland and recruited women with an IUGR fetus on ultrasound (US). Intrauterine growth restriction was defined as EFW of less than the 10th centile based on sonographic measurements of fetal biparietal diameter, head circumference, AC, and femur length. From January 2010 through June 2012, 1200 consecutive parturients with US-dated singleton pregnancies were recruited. Inclusion criteria were a gestational age between 24 0/7 and 36 6/7 weeks and an EFW of 500 g or greater. Adverse perinatal outcome was defined as a composite of intraventricular hemorrhage, periventricular leukomalacia, hypoxic ischemic encephalopathy, necrotizing enterocolitis, bronchopulmonary dysplasia, sepsis, and death. The outcomes were analyzed based on various IUGR definitions including EFW or AC of less than 10th, less than 5th, or less than the 3rd centile, with or without associated oligohydramnios and with or without abnormal umbilical artery (UA) Doppler results. The 3rd, 5th, and 10th centile cutoffs were used for EFW and AC and the presence of oligohydramnios or abnormal Doppler was considered as a predictor of adverse perinatal outcomes, neonatal intensive care unit (NICU) admission, and perinatal mortality.Of 1200 subjects, 1116 women completed the study. The mean maternal age was 30 years and mean gestational ages at enrollment and delivery were 30.1 and 37.8 weeks, respectively. One in 20 infants was affected by the composite adverse perinatal outcome, and 1 in 4 infants required NICU admission. Of the entire cohort with an EFW of less than the 10th centile, 800 fetuses (72%) had a normal outcome (ie, no NICU admission, morbidity, or death). Fifty-one (6.2%) of 826 infants with EFW of less than the third centile had composite adverse outcomes; 5 (2%) of 254 with EFW in the 3rd to 10th centiles had composite adverse outcomes. An abnormal UA Doppler indicated a significantly increased risk of adverse perinatal outcome. Estimated fetal weight or AC cutoffs of less than the 5th or less than the 10th centiles were not significantly linked to adverse perinatal outcome. The most significant association with adverse perinatal outcome was the presence of an abnormal UA Doppler. The only sonographic weight-related definition consistently associated with adverse perinatal outcome was an EFW of less than the third centile (P = 0.013). The presence of oligohydramnios was important only when combined with an EFW of less than the third centile (P = 0.007). A total of 312 infants required NICU admission with a median stay of 13 days. Better predictive ability of fetal biometry markers (EFW of less than the third centile, AC of less than 3rd or less than the 5th centile) was found for NICU admission in the absence of abnormal Doppler. Four stillbirths and 4 neonatal deaths occurred (7.2/1000 births); all fetuses had an EFW of less than the third centile.The optimal definition of growth restriction, especially the differentiation between SGA and IUGR, is one of the most common, controversial, and complex problems in obstetrics. Until patients identified with various cutoffs undergo a randomized or blinded trial, the answers regarding optimal IUGR definition, surveillance, and intervention cannot be clarified.