Title:
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Isolated oligohydramnios in low-risk pregnancy- a prospective study of the maternal, placental and fetal aetiological factors and associated perinatal outcomes
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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.
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