Background: There is an unsubstantiated conviction among clinicians that a significant reduction In amniotic fluid volume is a poor prognostic sign for pregnancy, even when it is an isolated finding. This belief has led to the inculcation of serial ultrasound assessments of amniotic fluid volume into the antenatal assessment of fetal well being with the aim of improving perinatal outcomes by closer monitoring and earlier delivery. In reality, there is no strong evidence base for this practice, while there are significant risks associated with premature delivery. There may also be cost and resource implications for the practice. Objective: The aim of this thesis was to investigate whether isolated oligohydramnios is significantly associated with adverse pregnancy outcomes in otherwise uncomplicated pregnancies, and to investigate the underlying associated maternal, fetal and placental aetiological factors. Materials and Methods: Amniotic fluid volumes were measured using ultrasound in 3328 low-risk pregnancies between 19 and 41 weeks. These were otherwise uncomplicated pregnancies recruited at the antenatal booking visit following a normal 1st trimester screening result (a negative 11-14 week combined nuchal translucency ultrasound and maternal serum-biochemistry fetal structural and chromosomal abnormality screening test). Maternal characteristics (age, parity, ethnicity, socioeconomic status, weight, smoking and alcohol consumption) during the course of the pregnancy, Placental characteristics (site, position and maturity), and Fetal renal blood flow parameters were also studied and analysed for possible associations. Results: Isolated oligohydramnios occurred more frequently with advancing maternal age and lower parity. However, maternal ethnicity, weight and socioeconomic status did not have any direct influence on the occurrence of isolated oligohydramnios and neither did the maternal life style factors studied. Increasing placental maturity was significantly associated with significantly reduced amniotic fluid but neither the placental site nor location was. There was a significant association between oligohydramnios and poor perinatal outcome as judged by meconium staining of amniotic fluid during labour, emergency Caesarean delivery for fetal compromise, an increased requirement for neonatal resuscitation and endotracheal intubation. In the population of pregnancies studied, there was no significant correlation between isolated oligohydramnios and a suspicious or pathological cardiotocogram during labour. However, a low birth weight less than 2500 g at birth, admission to the neonatal intensive care unit, a prolonged length of neonatal intensive care unit stay, and perinatal deaths were all significantly associated with oligohydramnios. No association was seen with either a low Apgar score or a low fetal arterial cord blood pH. Conclusion: This study showed that an isolated reduction in amniotic fluid volume even in an otherwise uncomplicated pregnancy is significantly associated with a poor perinatal outcome, and can therefore not be safely ignored. This finding justifies the continued assessment of amniotic fluid volumes.