This article focuses on hydronephrosis that is detected by means of antenatal ultrasonography (US). This method of surveillance detects a significant fetal anomaly in 1% of pregnancies, of which 20-30% of cases are genitourinary in origin, and 50% manifest as hydronephrosis. [1, 2, 3] If not for antenatal detection by US, many of these urologic anomalies would manifest, as they did in the past, later in life as pyelonephritis, symptomatic flank or abdominal pain, renal calculi, hypertension, or even end-stage renal disease.
The degree and laterality of hydronephrosis may depend on the stage of pregnancy and the underlying etiology. US can detect the fetal bladder and kidney by 15 weeks' gestation and distinguish a central echo (renal sinus) by 18-20 weeks.  At 20 weeks' gestation, the fetus is larger, and an anomaly is easier to detect. Antenatal hydronephrosis has received significant attention since antenatal US became a mainstream screening tool; however, management and treatment remain controversial in terms of patient outcome.
In addition, much of the controversy stems from diagnostic dilemmas and difficulties in ascertaining which lesions are obstructive and potentially harmful to the developing fetal kidney and other organ systems affected by renal function. In general, patients with obstructive uropathy that poses a significant risk of neonatal demise due to pulmonary hypoplasia may be considered candidates for antenatal treatment.
Once antenatal treatment is decided on, controversy remains regarding the efficacy of therapeutic intervention because of the limited knowledge of the underlying natural history and the difficulty of standardizing patient selection and determining appropriate outcome measures.  Furthermore, early diagnosis of hydronephrosis may cause significant parental anxieties during the rest of the pregnancy.
Source and continue to read: https://emedicine.medscape.com/article/1016305-overview