Although human milk is the preferred feeding for premature infants, preterm infants are less likely to receive breast milk than term infants. The proportion of infants breastfeeding is influenced by maternal infant feeding intention and demographic factors such as maternal smoking, education, age and socioeconomic status. The effects of gestational age at birth on duration of breastfeeding have been less studied than other determinants of breastfeeding.

A recent population-based Swedish study of 37 343 mothers found that mothers of preterm infants were at higher risk of weaning before the infant was 6 months (odds ratio (OR) 1.48; 95% CI 1.33 to 1.64) compared with mothers of term infants.1 To date, little evidence is available on breastfeeding rates for infants of different gestational ages. This study used national data to investigate the effect of gestation on initiation and duration of breastfeeding up to 6 months in a population with a high rate of initiation. There were low numbers of preterm infants in this community sample, so we have focused on near-term infants born at 35–36 weeks’ gestation and term infants born 37–39 weeks’ gestation in comparison with infants born at 40 weeks or later.

METHODS

The Longitudinal Study of Australian Children (LSAC) is being implemented by a large multidisciplinary research consortium led by the Australian Institute of Family Studies.2 During 2004, over 10 000 children and their families were recruited to the study from a sample selected from the Health Insurance Commission’s Medicare database.3 The sample is broadly representative of all Australian children (citizens and permanent residents) in each of two selected age cohorts: children born between March 2003 and February 2004 (infants, n = 5000) and children born between March 1999 and February 2000 (children aged 4–5 years, n = 5000).

Written informed consent was obtained for each participating child, and the study was approved by the Australian Institute of Family Studies Ethics Committee.

The main data collection for Wave 1 was a face-to-face interview with the parent who knew the child best (parent 1).3 In 97% of cases, parent 1 was the biological mother. The final response to the recruitment of children in the infant cohort was 57% of those families who were sent a letter by the Health Insurance Commission.

In this paper we analysed data from singleton children in the infant cohort (aged 6–12 months at interview) for whom the following data were available: breastfeeding initiation and breastfeeding status at 6 months, birthweight, gestational age at birth, method of delivery, maternal age, maternal smoking (“did you smoke in pregnancy”), maternal education level, and level of socioeconomic disadvantage of the geographical location of the child’s household.4 There were 3600 children in this subsample. Preterm was defined as <37 weeks’ gestation. We excluded multiple births as these are associated with both prematurity and low birth weight, which makes it difficult to disentangle the effect of multiple birth only.

Adjusted odds ratios were estimated using multivariable logistic regressions. All statistical analyses were conducted using Stata version 9.2.

RESULTS

In this sample, 4.9% of infants were preterm, most of these were 35–36 weeks’ gestation (3.3%). We stratified the term infants into 37–39 weeks (42.9% of sample) and ⩾40 weeks (52.2%). About 30% of infants were born by caesarean section and 16.3% of mothers smoked in pregnancy; other demographic variables can given in table 1.

The rate of initiation of breastfeeding was somewhat lower for infants who were 35–36 weeks’ gestation (88.2%) than 37–39 weeks (92.0%) and ⩾40 weeks (93.9%) (table 1). Although univariate analysis showed that preterm infants were less likely to initiate breastfeeding, the difference was not significant when adjusted for maternal smoking, age, education, level of socioeconomic disadvantage and caesarean birth (table 2).

Infants at earlier gestations were less likely to be breastfeeding at 6 months than infants born at 40 weeks or later (table 1). Of infants born at 35–36 weeks, 41.2% were breastfeeding at six months, compared with 54.5% of infants born at 37–39 weeks and 60.5% of infants born ⩾40 weeks. Figure 1 shows the more rapid cessation of breastfeeding in the 35–36 weeks’ gestation infants compared with infants born at later gestations.

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Figure 1 Duration of breastfeeding by gestation.

After adjusting for potential confounding factors (maternal smoking, age, education, level of socioeconomic disadvantage, caesarean birth) infants born at 40 weeks or later were still significantly more likely to be breastfed at 6 months than infants born earlier (table 2). Preterm infants born at 35–36 weeks had an adjusted OR of 0.51 (95% CI 0.34 to 0.76) of breastfeeding at 6 months compared with infants born ⩾40 weeks (table 2). Even term infants born at 37–39 weeks were significantly less likely to be breastfeeding at 6 months compared with infants born ⩾40 weeks (adjusted OR 0.80, 95% CI 0.69 to 0.93).

DISCUSSION

Few studies of preterm infants have measured duration of breastfeeding to 6 months.1 This nationally representative sample of Australian infants has found that preterm infants are less likely to be breastfed at 6 months after adjusting for the standard confounding factors. More interestingly, infants born at 37–39 weeks who would be considered full-term had lower rates of breastfeeding than infants ⩾40 weeks’ gestation.

It has been recognised that borderline (near-term) preterm infants are likely to experience difficulties with initiating and maintaining breastfeeding.5 Wight has described how the near-term infant may present with subtle immaturity: poor ability to clear normal lung fluid, increased incidence of apnoea, increased risk of hypothermia, neurological immaturity and immature oro-motor development.5 Near-term infants are at higher risk of hypoglycaemia, jaundice, dehydration, slow weight gain and hospital readmission.5

Wight includes infants of 37 and 38 weeks in her review of near-term infants.5 The results of this paper appear to confirm that infants between 37 and 39 weeks have more problems breastfeeding than infants born at 40 weeks or later, and are at risk of breastfeeding failure. We are unaware of other studies that have reported breastfeeding duration for infants born at term, but less than 40 weeks.

We cannot determine the reasons for poorer breastfeeding outcomes in infants born before 40 weeks from these data, but we believe it is important for clinicians to recognise that there may be unexpected adverse consequences when infants are born between 37 and 39 weeks. In settings where artificial feeding is the norm, it may be easier for mothers of these infants to turn to infant formula rather than persevering with breastfeeding.

CONCLUSIONS

Infants born prematurely and before 40 weeks are at greater risk of being artificially fed than infants born at 40 weeks or later, thus increasing their vulnerability to infection and future ill-health. Infants born at 35–36 weeks need individual assessment and care planning to improve their chances of being successfully breastfed. Clinicians should be aware that even for “term” infants, those born before 40 weeks are less likely to be successfully breastfed than those who have completed 40 weeks’ gestation.