Objective: Intrauterine growth restriction is a major risk factor for perinatal morbidity and mortality. Ultrasonic foetal biometry is an important tool to monitor foetal growth. Therefore, the quality of these biometry scans is vital to achieve good diagnostic accuracy. We assessed the quality of foetal biometry during a nationwide trial and explored its association with sonographer's characteristics. Methods: Four scans from every sonographer (n = 154), performed at 29 and 35 weeks gestational age were collected. Two assessors scored these scans according to a national audit system. A quality score ≥ 65% was considered ‘adequate’. We compared the quality scores per scoring criterion (i.e. foetal head measurements, abdominal circumference and femur length with regard to magnification, correctness of the plane and calliper placement) and gestational age. We analysed the associations between characteristics of the sonographers and their scores. In a subsample of scans of 30 sonographers we determined the interrater agreement on the quality scores given by the two assessors independently. Findings: The mean score was 81.3%. Thirteen sonographers (8.4%) failed to achieve ‘adequate quality’. Scores for femur length (83.8%) were significantly higher than those for head (77.9%) and abdominal circumference (78.6%) (both P < 0.05). Scores for correctness of the plane (73.4%) were lower than those for magnification (81.2%) and calliper placement (85.7%) (both P < 0.05). Gestational age did not affect the quality scores. Only the number of scans performed in the previous year was positively associated with the scores (β = 0.01; P < 0.05). The mean interrater difference in quality scoring was 11.1%, with 77.6% agreement on scans of ‘adequate quality’, but with no agreement on scans with ‘insufficient quality’. Key conclusions and implications for practice: Most sonographers achieved an ‘adequate quality’ score. Highest quality scores were attained for femur length, lowest quality scores for the correct plane. The number of scans one performs is associated with the quality scores, yet the minimum number of scans to perform for guaranteed quality still needs to be determined. Further research is needed to develop a standardized method to assess and maintain good ultrasonic foetal biometry quality.