OBJECTIVES: We studied the risk of emergency caesarean delivery (CD) using blinded ultrasonographic estimated fetal weight (EFW) at 36 weeks of gestational age (wkGA): (1) to compare the association for customised and non-customised EFW, (2) to determine whether adding ultrasonic EFW improved prediction based on maternal characteristics alone, and (3) to determine whether women at high predicted risk of emergency CD had higher risks of maternal and perinatal morbidity than other women. METHODS: We studied 3,047 low risk women (no pre-existing medical conditions or acquired complications of pregnancy) from the Pregnancy Outcome Prediction study (Cambridge UK) who had ultrasonic EFW at ~36 weeks gestational age, where women and clinicians were blinded to the result. RESULTS: Blinded EFW was strongly associated with the risk of emergency CD (coefficient for a 1 standard deviation increase in EFW = 0.39 [95% CI 0.30 to 0.48], odds ratio [OR] = 1.48 [95% CI 1.35 to 1.62]). The coefficient for customised EFW was similar (0.42 [95% CI 0.33 to 0.51], OR = 1.53 [95% CI 1.39 to 1.67]), hence, for simplicity, non-customised EFW was subsequently employed. Maternal characteristics (age, height, body mass index, and weight gain between 12 and 36 weeks) when combined in a multivariate logistic regression model were moderately predictive for emergency CD (AUROCC = 0.68). Adding blinded EFW to the model increased the AUROCC to 0.71 and this model was more predictive (P < 0.0001). When using this model and defining screen positive as a predicted risk of emergency CD ≥40%, 189 (6.2%) women screened positive and the proportion delivered by caesarean was 48%. Compared with screen negative women, they had elevated risks (relative risk [95% CI]) of severe postpartum hemorrhage (2.49 [1.83 to 3.38]), any adverse neonatal outcome (1.86 [1.22 to 2.82]), and severe adverse neonatal outcome (4.03 [1.35 to 12.03]). The risks of these events were also higher compared to women who had a term CD for breech presentation. The model was similarly predictive of the risk of emergency CD and perinatal morbidity when evaluated using routinely collected data from 55,337 births in Scotland between 2003 and 2008. CONCLUSIONS: Ultrasonic EFW at 36 weeks, combined with maternal characteristics, identifies women who are at increased risk of subsequent emergency CD. These women were at increased risk of maternal and perinatal morbidity compared with women at low risk of emergency CD and with women having CD for breech presentation at term.