Fig. 3 a.

to c Relationship between duration of pregnancy assessed using the Clearblue Digital Pregnancy Test with Conception Indicator and a ultrasound and b LMP; c shows the relationship between pregnancy duration assessed by ultrasound and LMP [27]. CAPT: Clearblue Advanced Pregnancy Test with Conception/Weeks Indicator; CRL: crown rump length; LMP: last menstrual period. A Bland-Altman plot of differences is a method to analyse agreement between two different, but highly correlated methods of measurement. The plot investigates the existence of any systematic difference between the clinical measurements. If the mean difference ± 1.96 SE (standard error, dotted line) of the two methods contains zero then the two methods can be used interchangeably.