br Methods br Results br Discussion Mean birth weight

Methods
Results
Discussion Mean birth weight was significantly higher among midwifery patients, in every moderate quality study in which it was examined (Heins et al., 1990; McLaughlin et al., 1992; Visintainer et al., 2000). Other studies have reported a birth weight gradient associated with maternal education, a common measure of SEP (Mortensen et al., 2008; Mortensen, Diderichsen, Smith, & Andersen, 2009). In a Danish study by Mortensen et al. maternal smoking was identified as the key mediator reducing infant birth weight for women with low education (Mortensen et al., 2009). The three studies in this review that found a significant positive association between midwifery care and heavier birth weights, controlled for smoking in their analyses. However, none of the studies measured smoking reduction or cessation over the course of pregnancy by practitioner-type, a factor that could have influenced the outcomes. This raises the question of self-selection bias, commonly suspected in midwifery/physician comparison studies, in which cohorts have systematically different health or behavioral characteristics associated with choice of caregiver. Four of the moderate quality studies demonstrated evidence of adjustment for self-selection bias. Both of the randomized controlled trials included in this review (Heins et al., 1990; McLaughlin et al., 1992) attained comparability between cohorts on all measured demographic characteristics, with the exception of marital status for primiparas in the study by McLaughlin et al., suggesting unknown confounders were likely controlled for through design. Benatar et al. utilized propensity score modeling to create a comparison group with almost identical observable characteristics to that of the midwifery cohort (Benatar et al., 2013). And, in the study by Simonet et al. there was likely little to no self-selection bias as all women were classified as midwifery or physician patients on the basis of their THZ1 manufacturer of residence, regardless of the actual maternity provider involved in care (Simonet et al., 2009). Of interest, in Fischler et al.’s study, a significant difference in average birth weights was reported between private practice midwifery patients and physician patients (191g, p<0.05), but not among midwifery patients serviced at a hospital-based clinic compared to physician patients—despite controlling for demographic and medical risk (Fischler & Harvey, 1995). In interpreting these differing results, Fischler et al. speculate that the model of care provided by midwives in a hospital setting may bear a greater resemblance to the medical model of care than to midwifery care, thus producing outcomes similar to those of physician-led care. Among reviewed studies that found an association between midwifery care and lower prevalence of adverse birth outcomes, three included women with more than one social or medical predictor of risk. In the study by McLaughlin et al. meaningful differences were found for average birth weight for midwives’ patients who were nulliparous and poor, compared to physicians’ patients (McLaughlin et al., 1992), but not for multiparous women who are at less risk of poor birth outcomes (Shah, 2010). Though these results should be viewed with caution because of a small sample size (n=165), they are in agreement with theory underlying other successful antenatal interventions aimed at lowering prevalence of adverse infant birth outcomes for low income women. For example, the Nurse–Family Partnership Program (Olds, Henderson, Tatelbaum, & Chamberlin, 1986) has traditionally only included first time mothers, as it is hypothesized that they are especially receptive to perinatal and lifestyle counselling (a major component of midwifery care), compared to multiparous women who may resist new advice in favor of deferring to previous personal experience (Olds, 1981). Secondly, Benatar et al. utilized a sample population comprised of 85% African American, low-income women, finding a significant improvement in PTB rates for midwifery patients (Benatar et al., 2013). In the U.S., women of African American race/ethnicity have higher prevalence of PTB, as do women of low-income (Martin & Osterman, 2013). Lastly, in a post hoc, sub-analysis Heins et al. found midwifery care to significantly lower VLBW only for African American women who had high medical and/or social risk scores (Heins et al., 1990).