Measurements of amniotic fluid volume are a vital component of fetal surveillance because both oligohydramnios and polyhydramnios are closely associated with impaired fetal growth and structural abnormalities. Noninvasive alternatives to amniocentesis rely on sonographic measurements of the amniotic fluid index (AFI) and the single deepest pocket (SDP), both of which correlate with amniotic fluid volume as measured invasively. This prospective observational study assessed the effects—if any—of fetal position on these sonographic parameters in 81 women with uncomplicated singleton pregnancies, all of whom had a longitudinal lie and cephalic presentation at or after 28 weeks' gestation. Fetal position was characterized by the side of the maternal abdomen on which the fetus lay; a laterality score reflecting the distance of the fetus from the sagittal midline plane of the maternal abdomen; and the orientation of the fetal trunk as being ventral anterior, lateral, or posterior. A single sonographer performed all the examinations.
Mean gestational age at the time of sonography was 33 weeks. Average values for SDP and AFI were 5.5 and 14.5 cm, respectively. Three women had polyhydramnios and one had oligohydramnios. The series included 34 fetal trunk right cases (42%) and 47 fetal trunk left cases (58%). These groups did not differ significantly in gestational age or mean SDP and AFI values. Laterality scores did not correlate significantly with SDP, but they did influence the AFI significantly; AFI increased in proportion to increased laterality scores. The AFI was more than 4 cm higher on average when the fetal trunk was at the midline of the maternal abdomen than when it lay on the side and did not cross the maternal midline. No significant differences were found in SDP, AFI, or gestational age between ventral anterior, ventral lateral, and ventral posterior fetuses. Because fetal position influences measurements of AFI, SDP may prove to be more useful of the two for estimating amniotic fluid volume.