Relaxin for cervical ripening and induction of labour

Not enough evidence that relaxin helps ripen the cervix and induce labour.

Relaxin is a hormone secreted by the placenta in the final stages of pregnancy to ripen the neck of the uterus (cervix) and prepare it for labour. It is used to induce labour and is either made from genetically combined human relaxin or from animal preparations. The role of relaxin treatment remains unclear but it is possible that it may relax the muscle tissue of the uterus and therefore not work as well as other methods of induction. The review of trials found there was not enough research to show the true effect of relaxin. More research is needed.

Authors' conclusions: 

The place of relaxin, either purified porcine or recombinant human, as an induction or cervical priming agent is unclear. Further trials are needed to estimate the true effect of relaxin within current clinical practice.

[Note: The three citations in the awaiting classification section of the review may alter the conclusions of the review once assessed.]

Read the full abstract...

Relaxin is a protein hormone composed of two amino acid chains. The role played by relaxin in human pregnancy and parturition is unclear. Its use and involvement as a cervical ripening agent has been debated since the 1950s. Because the main source of human relaxin is the corpus luteum of pregnancy much of the early work on induction of labour has focused on porcine or bovine preparations. With the advent of DNA recombinant technology human relaxin has become available for evaluation. Relaxin is thought to have a promoting effect on cervical ripening. Due to a possible inhibitory effect on human myometrial activity, relaxin may not be associated with the concomitant increase in the rate of uterine hyperstimulation seen with other induction agents. This is one of a series of reviews of methods of cervical ripening and labour induction using a standardised methodology.


To determine the effects of relaxin (purified porcine and recombinant human) for third trimester cervical ripening or induction of labour in comparison with other methods of induction.

Search strategy: 

We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (May 2003) and bibliographies of relevant papers. We updated this search on 13 August 2009 and added the results to the awaiting classification section.

Selection criteria: 

Clinical trials comparing relaxin used for third trimester cervical ripening or labour induction with placebo/no treatment or other methods listed above it on a predefined list of labour induction methods.

Data collection and analysis: 

A strategy was developed to deal with the large volume and complexity of trial data relating to labour induction. This involved a two-stage method of data extraction.

Main results: 

Nine studies were considered; five have been excluded and four included examining a total of 267 women. There were no reported cases of uterine hyperstimulation with fetal heart rate (FHR) changes in any of the studies. There was no evidence of a difference between the rate of caesarean section in those women given relaxin compared with placebo (15.3% versus 14.2%; relative risk (RR) 0.79, 95% confidence interval (CI) 0.42 to 1.50, 4 trials, 257 women). There was a reduction in the risk of the cervix remaining unfavourable or unchanged with induction with relaxin (21.9% versus 49.3%; RR 0.45, 95% CI 0.28 to 0.72, 3 trials, 371 women). There were no reported cases of uterine hyperstimulation without FHR changes.