This study was to determine the prevalence of unplanned pregnancy in a national sample of Portuguese puerperae and to define a risk profile for women with unplanned pregnancy. We also evaluated the independent effect of unplanned pregnancy in prenatal care utilisation and in the prevalence of preterm birth, low birth weight, and small for gestational age birth. Data was collected through a national survey proposed to the 50 major public Portuguese hospitals and answers were obtained from 41 hospitals, resulting in a sample of 1582 deliveries. Participants were classified as having a planned or unplanned pregnancy and were compared according to socio-demographic, behavioural, clinical, obstetric, and anthropometric characteristics. The prevalence of unplanned pregnancy was 39.4%. In the unplanned pregnancy group, single women aged less than 19 years and more than 34 years, those with one or more previous pregnancies, with 25% or less of the expected education for their age, and with the National Health Service as the source of were significantly more frequent. These characteristics, except the health provision, were significant and independently associated with an increased risk of unplanned pregnancy. Women with unplanned pregnancy were more likely to be unaware of prenatal care system, to have no prenatal care, late or inadequate prenatal care. Unplanned pregnancy was significantly associated with the risk of inadequate prenatal care (Odds ratio (OR) = 2.5; 95% Confidence interval (CI): 1.7-3.0), after adjusting for the effect of the confounding variables. Planning pregnancy was not significantly related with the prevalence of low birth weight or small for gestational age birth. However there was a significant relationship between unplanned pregnancy and preterm birth (Adjusted OR = 1.7; 95% CI:1.0-2.9). This study showed a high prevalence of unplanned pregnancy and that it was related with an under utilisation of prenatal care and with an independently increased risk of preterm birth, a major issue in perinatal health.