We categorized out-of-hospital and in-hospital births in Oregon according to the intended place of delivery and in comparing outcomes found that the risks for some adverse neonatal outcomes were increased among planned out-of-hospital births. In many previous U.S. studies, it was not possible to disaggregate planned in-hospital births from planned out-of-hospital births that took place in the hospital after a woman’s intrapartum transfer to the hospital.3,9,10 The latter births represent 16.5% of planned out-of-hospital births in our population, and misclassification of these births as in-hospital births caused rates of adverse outcomes among planned out-of-hospital births to be underestimated (in some cases, substantially).

We observed higher rates of perinatal deaths, depressed 5-minute Apgar scores, neonatal seizures, and maternal blood transfusions among planned out-of-hospital births; these persisted after multivariable and propensity-score adjustment. In other, similar studies in which it was not possible to account for intrapartum transfers to the hospital, results similar to ours were reported for neonatal deaths, neonatal seizures, and Apgar scores.3,6,7,9,18

Out-of-hospital births were also associated with a higher rate of unassisted vaginal delivery and lower rates of obstetrical interventions and NICU admission than in-hospital births, findings that corroborate the results of earlier studies.3-5 These associations follow logically from the more conservative approach to intervention that characterizes the midwifery model of care8,19 and from the fact that obstetrical interventions are either rare (e.g., induction of labor)20 or unavailable (e.g., cesarean delivery, whether at home or at a birth center) outside the hospital setting.

There are few current data available on rates of out-of-hospital-to-hospital transfer in the United States. The observed rate of 16.5% in this study is informative and is consistent with rates reported in a recent systematic review of transfers in developed countries (including the United States), in which intrapartum transfer rates ranged from 10 to 17%.21

The limitations of our study require consideration. First, a major limitation is the inability in the case of planned home births to distinguish between transfers from birth centers and transfers from home. Although there are important differences between these two settings,2 most state offices of vital statistics do not as yet distinguish between them in the case of transfers. Second, we controlled for maternal characteristics in regression models, but there are probably differences between women who choose to give birth in a hospital and those who choose out-of-hospital birth. Women who choose out-of-hospital birth have different values and goals for their delivery (e.g., control over surroundings and a nonmedicalized experience without unnecessary interventions) than do women who choose hospital birth (e.g., the availability of pain relief and access to emergency services).22 Third, although Oregon has a high out-of-hospital birth rate, the annual number of births in the state is relatively small (approximately 45,000, before exclusions), which provides low power for the analysis of rare outcomes. Our study was underpowered to analyze specific outcomes according to provider type, making this a useful area for future research. Fourth, since we analyzed data from only one state, it is hard to generalize our findings. Fifth, the accuracy of vital statistics data has well-known limitations, especially in regard to patient conditions before pregnancy; the coding of these conditions is less sensitive than that for procedures.23-26

Finally, misclassification or residual confounding may have affected our results. There are also differences in completion of birth certificates according to birth setting,2,25,27 and the accuracy of the reporting of many demographic and clinical variables is unknown. For example, the fact that 27 transfer patients are listed as having a physician as their planned birth attendant is most likely due to errors in birth-certificate completion; data are currently lacking to inform the degree of misclassification related to this and others factors that affect the study outcomes.

Out-of-hospital birth remains controversial. Studies from Europe have shown that out-of-hospital birth can be a safe option for women and their babies when the risk of complications is low.28-30 The European Union defines uniform standards for the education and training of midwives,31 whereas the United States takes a piecemeal approach to the training and credentialing of out-of-hospital birth attendants. The American College of Nurse-Midwives and the North American Registry of Midwives recommend that midwives should at minimum meet the standards of midwifery established by the International Confederation of Midwives (ICM), which include completion of a formal midwifery education program, national certification, and licensure in the local jurisdiction of practice.32,33 Certified professional midwives (CPMs) may achieve certification through apprenticeship and portfolio evaluation without obtaining a formal midwifery degree; within CPM professional organizations efforts are under way to uniformly adopt ICM standards.33,34 Oregon has followed this trend; in 2015 licensure became mandatory for attendants at out-of-hospital births.

The extent to which midwifery is integrated into a health care system probably explains some of the differences in practice and outcomes reported in U.S. and European studies. For example, the Dutch home-birth system (in which home birth is common and adverse outcomes are rare) includes formal collaborative agreements between out-of-hospital and in-hospital providers, clear and mutually agreed-upon stratification of risk, and protocols for the transfer of care.35,36 The process of devising evidence-based guidelines for U.S. home births is under way.37

Rates of obstetrical intervention are high in U.S. hospitals, and we found large absolute differences in the risks of these interventions between planned out-of-hospital births and in-hospital births.38 In contrast, serious adverse fetal and neonatal outcomes are infrequent in all the birth settings we assessed, and the absolute differences in risk that we observed between planned birth locations were correspondingly small; for example, planned out-of-hospital births were associated with an excess of less than 1 fetal death per 1000 deliveries in multivariate and propensity-score-adjusted analyses. Consideration of maternal preferences, including preferences for obstetrical services, is also important; the fact that U.S. hospitals generally decline to allow vaginal birth after a woman has undergone cesarean section may be associated with the increase in home births.10,39,40

Using data from Oregon birth certificates, we showed that the rates of obstetrical interventions were lower but the risks of perinatal death and other adverse neonatal outcomes were higher with planned out-of-hospital birth than with planned in-hospital birth; however, the absolute differences in the risks of adverse neonatal outcomes were small. Our findings highlight the effect that the misclassification of intended birth setting has on the accuracy of U.S. vital statistics.