Bladder and Kidneys
The fetal urinary system (kidneys, ureters, bladder, and urethra) develops from the same origin, but slightly ahead of, the reproductive system.
The bladder starts to develop around 9 weeks of pregnancy and can be seen on ultrasound as early as 10 weeks (and in 100% of fetuses by 13 weeks). The size of a normal bladder during the first trimester is less than 7 millimeters (mm).
During weeks 13 to 21, the bladder develops its specialized lining inside, and its muscle layer outside, while the bladder wall continues to increase in thickness throughout pregnancy.
As early as 13 to 15 weeks, the bladder can store and empty urine, and gets better at this process as pregnancy continues to progress.
Bladder size grows rapidly during the second half of pregnancy. At 20 weeks, the volume of the bladder is approximately 1 milliliter (ml); at 40 weeks, bladder volume can reach 36 to 54 ml, and urine production matches this increase in volume. Urine production is estimated to increase from 5 ml/hr at 20 weeks to 51 ml/hr at 40 weeks.
The bladder can be seen emptying approximately every 25 to 30 minutes at 28 weeks and every 60 minutes at 40 weeks. However, the bladder is never completely empty, and always contains some small amount of urine.
The fetal urinary system can produce urine at a rate of 500 to 1,200 ml/day at term, which is the major source of amniotic fluid in the second half of pregnancy. Fetuses with kidney problems can have a complete lack of amniotic fluid.
Note: In infants and children, the bladder is in the abdomen. It starts to enter the greater pelvic area around 6 years of age but does not actually enter the lesser part of the pelvis until puberty.
Three sets of successive kidneys develop in embryos. The first set is simply very basic, the second set only functions briefly, and the third set becomes the permanent kidneys.
The first set of kidneys appears early in the 6th week of pregnancy; very quickly, these degenerate except the ducts, which are used by the second set.
The second set appear late in the 6th week, and function as temporary kidneys for about four weeks, until the permanent ones develop and function.
Evidence of functioning kidneys (final set) may start as early as 12 to 15 weeks, when urine can be observed in the bladder. The kidneys themselves can be seen on ultrasound by 13 weeks, but are located much lower (near the pelvic region) in the body compared to kidney placement as an adult.
In some babies, the kidneys fail to ascend before delivery, and therefore remain in the pelvic region – known as pelvic kidney (about 1 in 1,000 births).
Nephrons are the “filtering units” of kidneys; each adult kidney contains about one million nephrons. Most nephrons in the kidney are formed during the second half of pregnancy.
Few studies from the past several decades have examined nephron development, but they are thought to be completed around 36 weeks, but may range from 32 to 37 weeks.
Kidney development ends near term, and no new nephrons are developed after birth. Due to this, it is currently assessed that the intrauterine environment may be the largest influential factor of the infant's kidney health, to include when the infant becomes an adult.
Women should ask their health care provider (HCP) any questions they have regarding their babies' bladder and kidney health, especially after an ultrasound examination. Although most women get their first full assessment between 12 and 14 weeks, some women may not get a detailed scan until 20 weeks, in which almost all components of the bladder and kidneys can be seen.