Effectiveness of Antenatal Anti-D Prophylaxis simple meta-analysis would be inappropriate, and the many methodological limitations in the studies would make interpreta- tion problematic. We addressed these issues by performing a bias- adjusted meta-analysis. Biases identified The potential internal biases affecting each of the 10 studies, as identified through completion of an internal bias checklist, are summarised in Table 2. Selection bias was a major concern in seven studies, since women receiving RAADP were compared to historical controls who were very likely to have differed from the intervention women in many respects, and the statistical analyses did not adjust for confounding. The lack of blinding (which would be difficult in this setting for ethical reasons) of both subjects and caregivers means that the chance of receiving directed anti-D after a potentially sensitising event possibly differed between interven- tion and control groups in all studies. In addition, there were losses to follow-up, although not all studies provided information on this, and there were problems with assessing sensitisation accurately in a few of the earliest studies. External biases arose from differences between the available RAADP studies and the pre-defined target setting with respect to population, intervention, control strategy and the outcome measure (Table 1). Figure 2. Biases in the Trolle study. (a) 67% ranges elicited from assessors A–D for additive internal biases, with means and 67% ranges for total internal bias; (b) 67% ranges elicited from assessors E–H for proportional external biases, with means and 67% ranges for total external bias. doi:10.1371/journal.pone.0030711.g002 PLoS ONE | www.plosone.org 6 February 2012 | Volume 7 | Issue 2 | e30711