Sometimes, there's no obvious reason for premature labour. Some possibilities are:
If you think you're in labour - because contractions start, your waters break or you're bleeding - contact the hospital.
If your labour is very early, you may be able to stall it for a while with drugs. Premature babies are at risk of respiratory distress syndrome (RDS); giving injections of corticosteroid drugs to the mum can reduce the risk of this happening, as they help the baby's lungs mature.
Sometimes, premature labour is induced. This might happen if it's thought the baby has a better chance of survival outside the uterus or if the mum has a threatening condition, such as severe pre-eclampsia or eclampsia.
In recent times doctors have been able to dramatically improve the survival hopes for babies born as early as 22-23 weeks. However, these infants face a huge battle at the start of life, with a risk of many significant long-term problems. And we still don't know in many cases why labour starts early or how to stop it.
Premature labour can develop in any woman but especially very young, single or unsupported mothers, and those who are underweight or smoke.
Infection is another potential trigger. Vaginal infections such as gonorrhoea, chlamydia, trichomonas and group B streptococci have all been linked to preterm labour.
A condition called bacterial vaginosis, which changes the acidity of the vagina, is also associated with preterm labour, possibly because it reduces the body's natural defences against infection.
It may therefore, be possible to reduce the risk by screening for and treating these infections with antibiotics. But this must be done before premature labour starts. Once labour begins it's too late.
In fact, that is the challenge with premature labour - to spot it before it gets going. The only absolute proof of labour is dilation of the cervix, and by then it's too late to stop the baby being born.
Contractions of the uterus can be confusing as many women experience contractions that may be painful, known as Braxton Hicks contractions, from 24 weeks of pregnancy. Two-thirds of women diagnosed as being in labour will not have delivered within 48 hours, and one in three continue their pregnancy to full term (the normal 40 weeks).
If a woman's membranes rupture, and she loses amniotic fluid, then labour becomes more likely especially as there is then a risk of infection. But urine may be mistaken for amniotic fluid and even special testing sticks can give incorrect answers.
Some research has examined chemicals that might give a clue that labour is about to start - one such is called foetal fibronectin, which is found in the secretions in the vagina. But so far these tests are no more than a rough guide. And of course such tests aren't much use when there is little you can do to stop a premature labour.
All sorts of things have been tried to halt premature labour, including drugs to stop the contractions of the uterus, ties to keep the cervix from opening and antibiotics to treat infection, but nothing is very effective.
Drugs that stop contractions (known as tocolytics) help in about a quarter of cases but rarely work for more than 48 hours and may have some risks, especially if the membranes have ruptured. These drugs are mostly used to postpone delivery until the woman can get to a hospital with a special care baby unit.
At the same time treatments are also given to try to prepare the baby for an early arrival, such as drugs to help mature the lungs. These may reduce the risk of the complications of prematurity, halving the severity of respiratory distress syndrome.
Babies born after 34 weeks only have a low risk of problems because their systems have almost completely matured, and labour is usually allowed to continue. But those under 28 weeks really need to be delivered in a hospital with a neonatal intensive care unit.
Babies born after only 23-24 weeks of pregnancy have a significant risk of long -erm problems.
In most cases there is little you can do to prevent preterm labour, but to reduce your risks: