It isn't so much that it causes malpresentations but that it leads them to persist. With rupture of membranes, a baby who is occiput posterior--the back of its head is against the mother's back instead of the more favorable anterior position towards her belly--can surge downwards into the pelvic inlet and become stuck in the OP position whereas with the cushion of forewaters, and more room to maneuver, it would likely have swiveled into the anterior position before descending. Simkin and Ancheta discuss this issue in the Labor Progress Handbook. I am not aware of formal research on this issue, but what we do know is that routine early AROM is not beneficial. It has a modest effect at best on shortening labor but no other benefits. On the contrary, it probably increases the risk of cesarean surgery (Smyth 2007), and it also creates the potential for umbilical cord prolapse. This being the case, there is no argument for doing it regardless of whether the theory that it increases risk of malpresentation is correct.
Smyth R, Alldred S, Markham C. Amniotomy for shortening spontaneous labour. Cochrane Database Syst Rev 2007(4):CD006167.
BACKGROUND: Intentional artificial rupture of the amniotic membranes during labour, sometimes called amniotomy or 'breaking of the waters', is one of the most commonly performed procedures in modern obstetric and midwifery practice. The primary aim of amniotomy is to speed up contractions and, therefore, shorten the length of labour. However, there are concerns regarding unintended adverse effects on the woman and baby. OBJECTIVES: To determine the effectiveness and safety of amniotomy alone for (1) routinely shortening all labours that start spontaneously, and (2) shortening labours that have started spontaneously, but have become prolonged. SEARCH STRATEGY: We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (30 March 2007). SELECTION CRITERIA: Randomised controlled trials comparing amniotomy alone versus intention to preserve the membranes. We excluded quasi-randomised trials. DATA COLLECTION AND ANALYSIS: Two authors assessed identified studies for inclusion. Both authors extracted data. Primary analysis was by intention to treat. MAIN RESULTS: We have included 14 studies in this review, involving 4893 women. There was no evidence of any statistical difference in length of first stage of labour (weighted mean difference -20.43 minutes, 95% confidence interval (CI) -95.93 to 55.06), maternal satisfaction with childbirth experience (standardised mean difference 0.27, 95% CI -0.49 to 1.04) or low Apgar score less than seven at five minutes (RR 0.55, 95% CI 0.29 to 1.05). Amniotomy was associated with an increased risk of delivery by caesarean section compared to women in the control group, although the difference was not statistically significant (RR 1.26, 95% CI 0.98 to 1.62).There was no consistency between papers regarding the timing of amniotomy during labour in terms of cervical dilatation. AUTHORS' CONCLUSIONS: On the basis of the findings of this review, we cannot recommend that amniotomy should be introduced routinely as part of standard labour management and care. We do recommend that the evidence presented in this review should be made available to women offered an amniotomy and may be useful as a foundation for discussion and any resulting decisions made between women and their caregivers.