Contraction frequency and the power of labour
The power of labour is how often and how strongly the uterus contracts, ideally about every 3 to 4 minutes in established first-stage labour (stronger and more regular contractions).
Remember
In established first-stage labour, regular contractions about every 3 to 4 minutes give adequate power; contractions that are too sparse, irregular or weak slow labour, while those that are too frequent risk foetal hypoxia (too little oxygen reaching the baby).
The three Ps help to work out why a labour is slow: the Passage (the birth canal, including the pelvic bones), the Passenger (the baby) and the power. Power means how often and how strongly the muscle in the wall of the uterus (the myometrium) contracts. Each contraction raises the pressure in the uterus, which helps the cervix (the neck of the womb) to efface (shorten and thin) and to dilate (open).
In established first-stage labour (after the slow early latent phase, when the cervix is about 4 cm open and contractions are stronger and more regular), contractions are ideally every 3 to 4 minutes. That is often enough to keep opening the cervix, yet it leaves a gap in which the uterine muscle relaxes. Once the cervix is fully dilated and the second stage begins, contractions ideally come slightly more often, about every 2 to 3 minutes.
When contractions are too infrequent, irregular or weak, they fail to open the cervix, and labour slows. An intravenous infusion of oxytocin (a hormone that makes the muscle in the wall of the uterus contract) can then be given to strengthen them. This is called augmenting labour (helping a slow labour along).
Too many contractions are just as unhelpful. More than 5 contractions in 10 minutes for at least 20 minutes is called tachysystole (excessively frequent contractions), and it is common while oxytocin is running. The rests become too short, so the baby is at risk of too little oxygen. While oxytocin is running, the foetal heartbeat and the contractions are monitored continuously, and the dose is reduced or stopped if there are more than 4 contractions in 10 minutes.
Key facts
- Established first stage
- regular contractions ideally come about every 3 to 4 minutes.
- Second stage
- contractions ideally come slightly more often, about every 2 to 3 minutes.
- Too infrequent or weak
- an intravenous oxytocin infusion can be given to strengthen contractions.
- Too frequent
- more than 5 contractions in 10 minutes for at least 20 minutes is tachysystole; if oxytocin is running it is reduced or stopped, and tocolysis (a medicine that slows contractions) may also be needed.
- Oxytocin safety
- a cardiotocograph (CTG, a belt that records contractions and the foetal heartbeat) runs continuously, and the oxytocin is reduced or stopped if there are more than 4 contractions in 10 minutes.
Exam traps
- Faster is not better: contractions every 1 minute are 10 contractions in 10 minutes, double the usual upper limit of 5, and the rests become too short for the uterus and baby to recover.
- A gap of about 10 minutes between contractions is typical of the very start of labour, so it describes early labour, not effective power once labour is established.
- Regular contractions are not enough on their own: once labour is established, contractions every 6 to 8 minutes are only one or two in 10 minutes, too sparse for adequate power, so frequency matters as well as regularity.
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