Size and volume charts of fetal bladder during the second and third trimester of pregnancy

An enlarged fetal bladder (megacystis) is a rather infrequent ultrasound
finding related to a wide spectrum of causes. Although the most common cause is
lower urinary tract obstructions (LUTOs), fetal megacystis is also associated
with more complex congenital malformations or chromosomal and genetic
anomalies, but it can also be a transitory finding, with eventually normal
bladder size and renal function at birth. The mechanism leading to megacystis
in LUTOs consists of a bladder neck obstruction caused by urethral atresia or
posterior urethral valves resulting in progressive urine retention in the
dilated fetal bladder. The natural history of LUTO is highly variable and it is
associated with high perinatal mortality (45%) and postnatal morbidity. In
light of variable etiology and outcome, the prenatal counseling of parents in
case of fetal megacystis is particularly challenging.
Between the 10th and 14th week, the definition of enlarged bladder is clear and
refers to a sagittal bladder diameter * 7 mm. Moreover, the measurement of the
sagittal diameter during this period has been found of help to discriminate
fetuses with chromosomal defects and those with LUTO.
On the contrary, after the 14th week the definition of megacystis is variable
and subjective. Most of the study considers as enlarged, a bladder with failure
to empty in 45 minutes. This definition implies research bias and currently
affects the reliability of the studies in literature. Furthermore, the absence
of reference ranges makes it not possible staging the degree of the bladder
enlargement and thus of the urinary obstruction, in order to properly manage
the foetuses with LUTO and conscientiously counsel the parents.

The main objective of the study is to construct reference curves for fetal
bladder diameters and volume in the second and third trimester of pregnancy.
Moreover we will investigate the variation in the dimensions of renal pelvis in
relation to the degree of bladder filling in healthy fetuses compared to 20
cases with mild pielectasy (defined as an antero-posterior renal pelvis between
5 and 10 mm).
Secondary objective will be to prospectively validate both the cut-off of
bladder enlargement derived from this study and the prediction model for
postnatal renal function designed by MEBLA ( NL54637.042.15) study, in 20
cases of megacystis diagnosed in subsequent years.

Cross-sectional study.

An ultrasound examination will be performed in a cohort of 300 pregnant women
with healthy fetuses, repeating three ultrasound examinations, each lasting
approximately 5 minutes at an interval of 20 minutes. All three examinations
will take place during the same appointment. The measurements are non-invasive
and not associated with any known risks for the pregnant woman or her fetus.

The measurements will be performed by 3D transabdominal ultrasonography:
A 3D sweep will be taken of te fetal bladder and stored digitally in the memory
of the ultrasound equipment ( Voluson E8 or Voluson E10, GE) . This lasts about
10 seconds.
On the sweep the following measurements will be performed
1) The fetal bladder longitudinal length (FBSL) obtained at the level of the
midline sagittal plane. The distance from bladder dome to bladder neck will be
collect in millimeters by placing the calipers on the inner borders of the
bladder wall. After three measurements, the longest diameters will be
collected.
2) Anteroposterior (AP) and transverse diameters will be measured from the
largest transversal view by placing the calipers on the inner borders of the
bladder wall.
3) Two measurements (anterior and posterior) of the bladder wall will be
obtained at the level of pubic bone.
4) The bladder volume will be calculated by the off line buil-in function
SonoAVC (Sonography-based Automated Volume Count) .
5) On the same 3D volume both renal pelvis will be measured following the same
method

Three Abdominal ultrasound examinations will be performed on the same day, each
one will last 5 minutes. No physical, psychological examinations and neither
particular diet or specific behavioural rules are required.