All about premature birth

About one-in-ten babies is now born prematurely and our scientific and medical understanding is increasing each day.

When is a baby considered premature?

A full-term pregnancy lasts for about 40 weeks from the first day of a woman's last menstrual period, but a delivery between 38 to 42 weeks is still considered full term.

When a baby is born at 37 weeks or before, the infant is considered premature and the birth preterm.

How do you calculate a premature baby's age?

A preemie's gestational age is the number of weeks completed in the womb at birth, as calculated from the mother's menstrual dates and the infant's maturational features at birth. The chronological age is the actual time since birth. Adjusted age is the chronological age corrected for the amount of prematurity. Adjusted age is commonly used when the subject is the baby's development, but chronological age is best for calculating the timing of healthcare visits and immunisations.

What are the causes of premature birth?

Known risk factors account for about only half of the cases of premature delivery. These include infections like urinary tract infections, respiratory illnesses and vaginal infections. That's why testing, using cultures, can be important. Treatment either before or during labour can prevent the spread of infection to the infant. Gum disease and undetected viral illnesses are also linked with increased rates of prematurity.

Multiple births

Twins, triplets and beyond are seldom carried to term. Twins have a 25 to 50 per cent chance of an early arrival, and the odds rise from there as the number of infants carried in the uterus increases.

Congenital irregularities

Ultrasound can help to identify infants with irregularities in development that may prompt an early delivery. If the foetus needs an intervention before the due date, a premature delivery may be planned.

Mother's conditions

Mums with uterine or cervical abnormalities; chronic illnesses such as kidney disease, pre-eclampsia/eclampsia (a pregnancy-related illness with high blood pressure) or diabetes; or a poorly functioning, bleeding or damaged placenta usually require early delivery of the baby. Delivery may be by Caesarean section (C-section) or induced for the well-being of the mother and/or the baby.

Other factors have been linked to a higher rate of preterm birth

These involve:

  • women who have had a previous premature delivery;

  • women who become pregnant six weeks or less after a previous birth;

  • women who are underweight at conception or who gain less than 9 kg/20 pounds during pregnancy;

  • very young mothers and women over 40;

  • women who work on their feet late into pregnancy or perform heavy labour or are subject to a lot of physical or emotional stress;

Myths about preterm birth

Most preterm mums and dads wonder what they did to cause an early delivery, and some feel guilt. In the vast majority of cases, there is nothing that could have been done to prevent an early birth. But myths continue and include:

  • sex in the third trimester

  • worrying

  • bad thoughts

  • arguments with your partner

  • air travel

  • bad food

  • swimming


About 20 per cent of premature infants are delivered electively (have a planned delivery) because of the mother's or infant's condition and to avoid the stress of labour; some births have labour induced. Another 30 per cent have a preterm delivery after the membranes rupture, and about half of all preterm births are delivered after the start of premature labour.

Labour is started or a Caesarian delivery proposed if the infant is in distress, termed foetal distress.

If the mother has a high-risk condition and/or a very premature delivery is imminent, she may be transferred to a high-risk centre for delivery. This will allow her needs and those of her infant, who will require very specialised care, to be met.

Preventing prematurity

Preventing all preterm births is not possible, but good prenatal care and good medical care between pregnancies can help to lower the numbers.

Women with a high-risk condition or with any chronic or acute health condition should see their healthcare provider early and often. Good nutrition, appropriate weight gain and no smoking or drug use will also reduce the preterm rate, as will better diagnosis of subtle infections.

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