**2. Amniotic fluid**

The volume of amniotic fluid increases during pregnancy (Queenan et al., 1972). In general, the secretion of liquid by the kidneys and from the fetal lungs and oro-nasal cavity is balanced by the removal of equal amounts of liquid (Flack et al., 1995). The main clearance pathway is the swallowing of fluid by the fetus. Additionally, albeit to a lesser degree, fluid passes from the amniotic lumen via the surfaces of the placenta and umbilical cord into the fetal blood circulation (the intramembranous pathway) and into the mother's circulation (the transmembranous pathway) via the uterine wall through the surface of the amniotic sac outside the placental border.

#### **2.1 Abnormal amounts of amniotic fluid**

Oligohydramnios (reduced volume < 300 mL) is found in 3-5% (Hansmann, 1985; Volante et al., 2004). Rupture of the membranes is the most common cause of oligohydramnios. A reduction in amniotic fluid volume is of particular concern when it occurs in conjunction with structural fetal anomalies, fetal growth restriction, kidney abnormalities, postdate pregnancies and maternal disease. In these high-risk conditions, it is associated with a poor perinatal outcome (Camanni et al., 2009; Hill et al., 1983). Early onset of oligohydramnios

Renal Function and Urine Production in the Compromised Fetus 135

In cases with abnormal amniotic fluid volume, prenatal ultrasonography has been recommended for the evaluation of fetal anatomy and growth, swallowing patterns, blood

During the filling phase, the increasing volume of the urinary bladder can be observed,

Fig. 2. This figure shows an appropriate longitudinal bladder image. The 2D ultrasound image on the ultrasound screen was documented on a CD and the volume was calculated in

(Deutinger et al., 1987; Shin et al., 1987; Takeuchi et al., 1994).

The Hourly Fetal Urine Production Rate (HFUPR) can be estimated by regression analysis of calculated bladder volumes documented at different time points within one filling phase (Campbell et al., 1973; Fagerquist et al., 2001; Groome et al., 1991; Nicolaides et al., 1990; Rabinowitz et al., 1989; van Otterlo et al., 1977; Wladimiroff and Campbell, 1974), or by the difference between the maximum and minimum volumes divided by the time interval

flow velocity in different vessels and repeated estimation of the amount of fluid.

**3. Fetal urine production** 

a computer.

documented and assessed by ultrasound scans.

adversely affects fetal lung development, resulting in pulmonary hypoplasia, which might lead to death from severe respiratory insufficiency (Nicolini et al., 1989). However, numerous factors complicate the ultrasonographic diagnosis of oligohydramnios. They include the lack of a complete and detailed understanding of the physiology of the dynamics of oligohydramnios. For example, in 40%, the oligohydramnios occurs without any high-risk conditions and the current available data support the expectant noninterventional management of these cases complicated by isolated oligohydramnios (Sherer, 2002).

Data: Gilbert WM och Brace RA. Amniotic fluid volume and normal flows to and from the amniotic cavity. Semin Perinatol. 1993; 17: 150-157

Fig. 1. The amniotic fluid turnover at term. Half the secreted liquid from the fetal lungs and oro-nasal cavity reaches the amniotic sack and the other half is swallowed. The clearance pathways are denoted in italics.

Polyhydramnios (increased volume > 2,000 mL) is found in 1-3% (Volante et al., 2004). The underlying cause of excessive amniotic fluid volume is obvious in some clinical conditions in some clinical conditions and in cases of an minor an minor increase in amniotic fluid volume, the perinatal outcome is good. However, maternal kidney disease, diabetes type 2 and fetal conditions, such as chromosomal abnormalities, most commonly trisomy 21, followed by trisomy 18 and trisomy 13, might be causes (Hill et al., 1987). Moreover, polyhydramnios can be the result of oesophageal atresia and defects in the fetal CNS (Barkin et al., 1987; Kimble et al., 1998).

In cases with abnormal amniotic fluid volume, prenatal ultrasonography has been recommended for the evaluation of fetal anatomy and growth, swallowing patterns, blood flow velocity in different vessels and repeated estimation of the amount of fluid.
