Fetal hydronephrosis 23mm is more serious if it is accompanied by other congenital renal structural abnormalities. If the hydronephrosis is simple, it may also be caused by physiological reasons, which is not necessarily serious, so the pregnancy can be continued and the fetus can be further examined after birth. Fetal hydronephrosis is more serious if it is accompanied by other congenital anomalies of kidney structure, such as bilateral polycystic kidneys, renal dysplasia, etc. Sometimes, it may be accompanied by low amniotic fluid, etc., which need to terminate the pregnancy in time. In the case of simple hydronephrosis, it may be caused by physiologic reasons, such as a large amount of urine filling in the bladder of the fetus or malformation or narrowing of the ureter caused by some factors, and the hydronephrosis may shrink as the fetus grows. It may also be due to the presence of certain anatomical abnormalities in the fetal ureter. The pregnant woman may continue to observe and undergo further examination and treatment after the birth of the fetus. If the fetus has hydronephrosis, the pregnant woman should consult a doctor for further examination and follow the doctor’s instructions.