I am pregnant and I did ultrasound and doctor told me that there is a small swelling in baby's right kidney. please tell me the cause and what should I do. I am worried.

Question

Antenatal (before birth) hydronephrosis (fluid-filled enlargement of the kidney) can be detected in a fetus by ultrasound as early as the first trimester of a pregnancy. Depending upon the abnormality, ultrasound imaging may be needed throughout pregnancy and after a baby is born.
Antenatal (before birth) hydronephrosis (fluid-filled enlargement of the kidney) can be detected in a fetus by ultrasound as early as the first trimester of a pregnancy. During pregnancy, this condition is identified in 1 percent of males and 0.5 percent of females. Typically, this condition is not associated with abnormalities in other organ systems.
Prenatal intervention is almost never required, and amniotic fluid is usually normal. Depending upon the abnormality, ultrasound imaging may be needed throughout pregnancy and after a baby is born. In most cases, this diagnosis does not affect when, where or how a baby is delivered. Surgery is required in a small percentage of children during infancy and childhood.
What causes antenatal hydronephrosis?
This is the most commonly detected condition on prenatal imaging. This prenatal ultrasound image shows minimal renal pelvic dilation of less than 5 mm in both kidneys.
The most common site is where the renal pelvis joins the ureter (the duct by which urine passes from the kidney to the bladder). Ureteropelvic junction obstruction (UPJ) is estimated to be present in one in 1,000 infants.
Obstruction may also occur where the ureter joins the bladder, known as ureterovesical junction obstruction (UVJ) or megaureter. The incidence of this condition is one in 2,500 infants and more than 90 percent of these cases improve without surgery.
As result, the kidney cannot produce urine and does not develop normally. The kidney has no function. Fortunately, this usually only affects one kidney. Given that the other kidney is normal (and compensates for the absence of another functioning kidney), infants with a multicystic kidney are usually born with entirely normal overall kidney function.
Testing and treatment during pregnancy
In nearly all instances of antenatal hydronephrosis, ultrasound monitoring is all that is necessary. For most cases, a pregnancy is not affected, and a normal delivery can be performed. In the rare fetus with severe obstruction of both kidneys and insufficient amniotic fluid, prenatal intervention to relieve the obstruction is a consideration. Evaluation for possible intervention requires multiple specialties such as neonatology, pediatric urology and maternal-fetal medicine.
Testing and treatment following birth
Your baby may be placed on a low-dose, once-a-day antibiotic to prevent urinary tract infection.
Since an ultrasound performed in the first few days after your baby is born may underestimate the degree of this condition, the first ultrasound is usually conducted following discharge from the hospital.
However, there are circumstances when an ultrasound will be conducted prior to your baby’s discharge. This may be necessary because of:
bilateral dilation
decreased amniotic fluid
complications following birth
uncertainty regarding the ultrasound diagnosis.

Potnuru Srinivaasa Sudhakar · Accepted Answer

Antenatal (before birth) hydronephrosis (fluid-filled enlargement of the kidney) can be detected in a fetus by ultrasound as early as the first trimester of a pregnancy. Depending upon the abnormality, ultrasound imaging may be needed throughout pregnancy and after a baby is born.
Antenatal (before birth) hydronephrosis (fluid-filled enlargement of the kidney) can be detected in a fetus by ultrasound as early as the first trimester of a pregnancy. During pregnancy, this condition is identified in 1 percent of males and 0.5 percent of females. Typically, this condition is not associated with abnormalities in other organ systems.
Prenatal intervention is almost never required, and amniotic fluid is usually normal. Depending upon the abnormality, ultrasound imaging may be needed throughout pregnancy and after a baby is born. In most cases, this diagnosis does not affect when, where or how a baby is delivered. Surgery is required in a small percentage of children during infancy and childhood.
What causes antenatal hydronephrosis?
This is the most commonly detected condition on prenatal imaging. This prenatal ultrasound image shows minimal renal pelvic dilation of less than 5 mm in both kidneys.
The most common site is where the renal pelvis joins the ureter (the duct by which urine passes from the kidney to the bladder). Ureteropelvic junction obstruction (UPJ) is estimated to be present in one in 1,000 infants.
Obstruction may also occur where the ureter joins the bladder, known as ureterovesical junction obstruction (UVJ) or megaureter. The incidence of this condition is one in 2,500 infants and more than 90 percent of these cases improve without surgery.
As result, the kidney cannot produce urine and does not develop normally. The kidney has no function. Fortunately, this usually only affects one kidney. Given that the other kidney is normal (and compensates for the absence of another functioning kidney), infants with a multicystic kidney are usually born with entirely normal overall kidney function.
Testing and treatment during pregnancy
In nearly all instances of antenatal hydronephrosis, ultrasound monitoring is all that is necessary. For most cases, a pregnancy is not affected, and a normal delivery can be performed. In the rare fetus with severe obstruction of both kidneys and insufficient amniotic fluid, prenatal intervention to relieve the obstruction is a consideration. Evaluation for possible intervention requires multiple specialties such as neonatology, pediatric urology and maternal-fetal medicine.
Testing and treatment following birth
Your baby may be placed on a low-dose, once-a-day antibiotic to prevent urinary tract infection.
Since an ultrasound performed in the first few days after your baby is born may underestimate the degree of this condition, the first ultrasound is usually conducted following discharge from the hospital.
However, there are circumstances when an ultrasound will be conducted prior to your baby’s discharge. This may be necessary because of:
bilateral dilation
decreased amniotic fluid
complications following birth
uncertainty regarding the ultrasound diagnosis.