Bland–Altman Plot: Agreement Between Ultrasound-measured Placenta Thickness and Other Biometric Parameters in the Determination of Gestational Age

PURPOSE: The aim of this article is to evaluate the agreement of placenta thickness (PT) with other foetal biometric parameters in the determination of gestational age (GA) in normal singleton foetuses. MATERIALS AND METHODS: The study was a cross-sectional descriptive study conducted among 406 consecutively recruited pregnant women with singleton foetuses at 15–40 weeks of gestation at the National Hospital, Abuja, Nigeria from October to December 2019. Biparietal diameter (BPD), femur length (FL), head circumference (HC), abdominal circumference (AC), and PT were measured using standard measurement protocols. Bland and Altman (BA) plots were used to compare PT and other foetal biometric parameters. The significant statistical level was determined at a critical value of P < 0.05. RESULTS: The mean age of study participants was 31.8 ± 4.8 years. The BA plot of PT and HC demonstrated a normal distribution; the mean difference was around zero (3.968) and 95% of the measurements fell within 2SD of the mean. The BA plot of HC and AC measurements also showed that the 95% limits of agreement for differences fell within 10% of the mean of the measurements (-4.236 to 15.987 with a mean difference of 5.876), indicating good agreement between the two pairs of variables. However, BA plots between PT and BPD as well as PT and FL showed no agreement. CONCLUSION: This study indicates that there is good agreement or comparability between PT and HC measurements as well as between PT and AC measurements. Hence, either HC or AC measurements may be interchangeable with PT measurements in the ultrasound determination of GA. However, PT measurements did not agree well with BPD and FL measurements, respectively.