Main findings

Low-risk primiparous women intending to give birth in an FMU rather than in an OU were found to have a significantly higher incidence of a spontaneous, uncomplicated birth with good outcomes for mother and infant and significantly lower risk of intrapartum caesarean section. These women were furthermore significantly less likely to require labour augmentation, epidural analgesia, instrumental delivery or hospital readmission, while they were more likely to have a water birth. We found no indication that parity modified the effect of birthplace on the maternal and perinatal birth outcomes under investigation. As for transfer rates, substantial differences were observed between primiparous and multiparous women (36.7 and 7.2%, respectively), with slow progress of labour being the most frequent reason, irrespective of parity.

Strengths

An overall strength of the study is that all participating units operated under identical practice guidelines, in a publicly funded health care system with midwives as primary care providers for all low-risk births, thus minimising confounding by differences across birth settings in clinical practice, care provision and economic determinants of birthplace. Similarly, confounding due to differences in obstetric risk factors was of little concern as all the included women were assessed to be at low risk at the start of care in labour.

Moreover, a major strength of this study is the inclusion of all eligible women admitted to the participating FMUs during the study period, with no loss to follow-up, thereby providing a complete, high-quality data set.

Limitations

The non-randomised design represents an important limitation of the study. Despite the close matching on potential confounding factors and the restriction to low-risk women, the possibility of residual confounding cannot be excluded given the observational study design.

Further, an unexpected closure of the two participating FMUs posed a major challenge to the original study, preventing the harvest of the originally intended data amount. However, post hoc recalculation revealed only a modest loss of statistical power. Additional details are described in previous publications [37, 48]. As for the subgroup analysis presented here, primiparous women constituted only 25.6% of all participants; it may thus be questioned whether the statistical power of our study is sufficient to detect true differences between subgroups. The risk is nonetheless considered low, as the confidence intervals are relatively narrow.

A further potential limitation is the age of our dataset. However, the rigorous assessment criteria used for low obstetric risk are in line with the more recent and internationally accepted NICE guidelines [55] and may thus be considered up-to-date. The potential confounders, such as maternal age and pregestational BMI, have moreover maintained a steady level among women in the region [57, 58].

Interpretation of findings

Patient safety has been a central issue in debates over care in FMUs [32–35], with primiparas attracting special attention due to their relatively high rate of obstetric complications and transfers. Our study found no significant effect differences by parity for any of the investigated maternal and perinatal outcomes, indicating that FMUs serve primiparous and multiparous women equally well.

Freestanding midwifery units aim to offer low-risk women a choice among birth places without compromising safety for themselves or their infants. Severe complications and adverse outcomes are, however, difficult to measure in studies comparing different birth settings, as their occurrence is rare in women at low obstetric risk. The use of a strictly positive composite outcome allowed us to capture rare events. The finding that intention to give birth in an FMU rather than in an OU more than doubled their chance of having a spontaneous, uncomplicated birth with good outcome for mother and infant should be reassuring for primiparous women. Optimal positive outcomes of birth are thus entirely attainable for women attracted by alternatives to birth in a traditional OU setting.

For primiparous women planning to give birth in an FMU, the likelihood of intrapartum caesarean section was 60% lower than for primiparas planning to give birth in an OU. This result is consistent with the findings of the extensive Birthplace in England study and the research on freestanding midwifery units in Quebec [8, 44]. The result is likewise in line with the reduced use of interventions for all women generally documented by studies of care in FMUs [59]. It is increasingly being recognised that the mode of delivery in the first birth influences delivery mode and outcomes in subsequent births [19, 49–53, 60, 61]. This strongly suggests that the key to curbing the increasing use of caesarean section lies in preventing the need for first-birth caesarean deliveries [20–23, 25, 26]. This stresses the importance of our finding of a substantial reduction in caesarean section among primiparous women.

The main difference between primiparous and multiparous women was found in their respective transfer rates. Our finding of markedly higher rates for primiparas than for multiparas corroborates earlier work [36, 45, 46]. The studies cited also agree with our finding that the most frequent indication for transfer in both primiparous and multiparous women was a slow progress of labour. For safety reasons, the criteria for transfer on the indication of slow labour progress were rather strict in the participating FMUs (no progress for 2 h, see Fig. 3), which is likely to have contributed to the high transfer rate on this indication.

This study is the first to report the finding of a steep decline in overall transfer rates for primiparous women during the relatively short period of 2.5 years. As this occurred simultaneously with a decline in transfers of primiparous women on indication of slow progress of labour, we find this development likely to reflect the increasing experience gained by midwives in supporting normal birth and the positive influence of feedback from regular multidisciplinary audits with attendance of both FMU and OU staff. Another contributing reason may be that part of the FMU 2 data were collected from its opening day and thus during the units start-up phase, in which it may have been underperforming.

The decline in transfer rates was not associated with a concomitant increase in the incidence of Apgar score <7 at 1 min and postpartum haemorrhage >500 ml. In general, our results on maternal and perinatal outcomes suggest that the referral and transfer system represents an effective safety net to support both primiparous and multiparous women wishing to give birth in an FMU.

Overall, the present study adds to the limited body of evidence concerning the suitability of care in FMUs for primiparous women. Our findings may be relevant to birthing women as well as to health professionals and policy makers in the planning of maternity care services. The results of this and similar research suggest that, regardless of parity, care in an FMU is a safe alternative to care in an OU for low-risk women, and that care in FMUs offers important benefits for both primiparous and multiparous women. Furthermore, the reported reduction in caesarean delivery, both overall and in primiparous childbirth, indicates that FMUs may hold an untapped potential to halt or even reverse the global rise in the use of caesarean section.

Attempts to extend the validity of our results to other populations and regions should be regarded with circumspection. The participants in this study were drawn from an ethnically and culturally homogenous population of women with free access to all national maternity care services and; our results may therefore not be directly applicable to relatively more diverse populations. The quality and safety measures implemented in the participating units should also be taken into account. We would emphasize the importance of the training and experience of FMU midwives in managing obstetric emergencies, the high standard of transfer guidelines and the regular audits between FMU and OU staff on the quality of care.