A historical cohort study was conducted to examine the pregnancy outcome in women aged 40 or older and determine the effect of age on low birth weight. The pregnancy outcomes of 789 mothers aged 40 years or older were analysed and compared with those of 20,852 mothers aged 20-34 years. There were differences in socioeconomic status and obstetric characteristics between the two groups. The older group had more medical and obstetric complications (diabetes mellitus, chronic hypertension, malpresentation, pregnancy-induced hypertension, placenta praevia, multiple pregnancies, pre-term labour, fetal distress, retained placenta, postpartum haemorrhage and endometritis), more adverse fetal outcomes (low birth weight, low Apgar scores and congenital anomalies) and a higher caesarean section rate. The multivariate logistic regression analysis confirmed that maternal age was an independent risk factor for low birth weight. These data will be useful in counselling patients about their expectations and the risk of adverse outcomes and in providing the appropriate necessary care.