Objective
Warm water immersion during labour provides women with analgesia and comfort. This cohort study aimed to establish among women using intrapartum water immersion analgesia, without antenatal or intrapartum risk factors, whether waterbirth is as safe for them and their babies as leaving the water before birth.

Design
Cohort study with non-inferiority design.

Setting
Twenty-six UK NHS maternity services.

Sample
A total of 73 229 women without antenatal or intrapartum risk factors, using intrapartum water immersion, between 1 January 2015 and 30 June 2022. The analysis excluded 12 827 (17.5%) women who received obstetric or anaesthetic interventions before birth.

Methods
Non-inferiority analysis of retrospective and prospective data captured in NHS maternity and neonatal information systems.

Main outcome measures
Maternal primary outcome: obstetric anal sphincter injury (OASI) by parity; neonatal composite primary outcome: fetal or neonatal death, neonatal unit admission with respiratory support or administration of antibiotics within 48 hours of birth.

Results
Rates of the primary outcomes were no higher among waterbirths compared with births out of water: rates of OASI among nulliparous women (waterbirth: 730/15 176 [4.8%] versus births out of water: 641/12 210 [5.3%]; adjusted odds ratio [aOR] 0.97, one-sided 95% CI, −∞ to 1.08); rates of OASI among parous women (waterbirth: 269/24 451 [1.1%] versus births out of water 144/8565 [1.7%]; aOR 0.64, one-sided 95% CI −∞ to 0.78) and rates of the composite adverse outcome among babies (waterbirth 263/9868 [2.7%] versus births out of water 224/5078 [4.4%]; aOR 0.65, one-sided 95% CI −∞ to 0.79).

Conclusion
Among women using water immersion during labour, remaining in the pool and giving birth in water was not associated with an increase in the incidence of adverse primary maternal or neonatal outcomes.