Abstract: We included three trials involving 254 adolescent girls with type 1 or type 2 diabetes, with an overall unclear to high risk of bias. The three trials were conducted at diabetes clinics in the USA, and assessed the READY-Girls (Reproductive-health Education and Awareness of Diabetes in Youth for Girls) programme versus standard care. Considering primary outcomes, one trial reported no pregnancies in the trial period (12 months) (very low-quality evidence, with downgrading based on study limitations (risk of bias) and imprecision); in the other two trials, pregnancy was an exclusion criterion, or was not clearly reported on. None of the trials reported on the other primary maternal outcomes, hypertensive disorders of pregnancy and caesarean section; or primary infant outcomes, large-for-gestational age, perinatal mortality, death or morbidity composite, or congenital malformations. Similarly, none of the trials reported on the secondary outcomes, for which we had planned to assess the quality of the evidence using the GRADE approach (maternal: induction of labour; perineal trauma; gestational weight gain; long-term cardiovascular health; infant: adiposity; type 1 or 2 diabetes; neurosensory disability). The majority of secondary maternal and infant outcomes, and outcomes relating to the use and costs of health services were not reported by the three included trials. Regarding behaviour changes associated with the intervention, in one trial, participants in the preconception care group had a slightly higher score for the actual initiation of discussion regarding preconception care with healthcare providers at follow-up (nine months), compared with those in the standard care group (mean difference 0.40, 95% confidence interval -0.02 to 0.82 (on a scale of 0 to 4 points); participants = 87) (a summation of four dichotomous items; possible range 0 to 4, with 0 being no discussion). There are insufficient RCT data available to assess the effects of preconception care for diabetic women on health outcomes for mothers and their infants. More high-quality evidence is needed to determine the effects of different protocols of preconception care for diabetic women. Future trials should be powered to evaluate effects on short- and long-term maternal and infant outcomes, and outcomes relating to the use and costs of health services. We have identified three ongoing studies that we will consider in the next review update.

Plain-language summary: We found three randomised controlled trials, conducted at diabetes clinics in the USA. The total number of participants in the studies was 254. The participants were all adolescent girls involved in the programme READY-Girls (Reproductive-health Education and Awareness of Diabetes in Youth for Girls). Their care was compared with standard care. None of these three trials gave us the information on the health outcomes we needed. In one trial, there were no pregnancies among the participants during the period of the study, and the other two trials’ reporting of pregnancy was not sufficient. There were no data about short and long term outcomes for the mothers and their babies, or about the use of the health service and related costs. Because the information is lacking, we have no evidence from this Cochrane review to guide practice on this topic. Further large, well-designed, randomised controlled trials are required. Three trials are ongoing and will be considered in the next update of this review.