OBJECTIVE: The primary objective of our study is to compare the effects of routine hydration with additional 2 liters of normal water versus orally 2 liters of distilled water on the amount of amniotic fluid.

HYPOTHESIS: İsolated oligohydramnios, secondary to depleted maternal intravascular volume, can be reversed better with orally hypotonic water.

BACKGROUND: Olygohydramniosis is a condition in which amniotic fluid levels are lower than expected for gestational age. It is typically diagnosed by ultrasound examination and may be described qualitatively (e.g., reduced amniotic fluid volume) or quantitatively (e.g., amniotic fluid index ≤5 cm, single deepest pocket <2 cm). Water hydration aims to decrease maternal osmolality and sodium concentration, increase water flow osmotically from maternal blood to fetal amniotic fluid, and improve uteroplacental perfusion.

A meta-analysis comparing maternal hydration's effect reported that maternal intravenous hydration had good results on AFI in pregnancies with isolated oligohydramnios and hypotonic solutions were more effective than isotonic fluids(1). Maternal oral hydration may slightly increase amniotic fluid and is safer than intravenous fluid administration or amnioinfusion.

Reducing the osmolality in maternal blood with oral hypotonic and isotonic fluids helps to direct the placental water passage towards the fetus, to improve uteroplacental perfusion, and as a result, to increase the amount of amniotic fluid in pregnant women with isolated oligohydramnios. Osmolality is further reduced by maternal oral hypotonic fluid, thus improving the amount of amniotic fluid.

The study protocol was approved by the Local Ethics Committee, and written informed consent will be taken from patients entering the study. The sample size was calculated based on the "Maternal hydration increases amniotic fluid index" study of Kilpatrick et al.(2). When 1.5 cm increase in AFI is considered significant, with 90% power and type 2 error (alpha): 0.05, the sample size is 20 for each group, and it is planned to include 40 pregnant women in total.

I

Inclusion Criteria:

• Singleton pregnancy
• Gestational age (GA) 28-39 weeks (based on the last menstrual period and confirmed by the results of ultrasound or determined through early pregnancy sonography)
• Isolated oligohydramnios cases
• Amniotic fluid index ≤ 5
• No pregnancy complications such as preeclampsia, gestational hypertension
• Intact fetal membranes.

Exclusion Criteria:

• Presence of fetal complications (membrane rupture, presence of fetal anomaly, intrauterine growth restriction)
• Presence of maternal systemic disease (nephropathy, cardiac disease)
• Multiple pregnancies
• Conditions in which the amount of amniotic fluid cannot be accurately assessed (morbid obesity)