Amniotic fluid (AF) is a clear, yellowish fluid that surrounds the baby inside the uterus, and is an important part of the overall health assessment of a pregnancy.
In very early pregnancy, AF volume increases faster than the embryo and can be identified first via ultrasound.
Overall, AF composition changes with gestational age but also contains carbohydrates, proteins, lipids, urea, lactate, electrolytes, enzymes, and hormones, all of which contain valuable information on the health of the fetus.
Amniocentesis, the removal and testing of AF, has been a valuable tool in assessing fetal health since the 1970s.
AF has been investigated as a source of stem cells with initial positive results. However, it has not been determined whether certain valuable components in AF could survive pasteurization, freezing, and/or storage to be of any future health benefit, and research remains ongoing.
AF may also contain substances that are potentially harmful:
AF has traditionally been viewed as sterile, but bacteria has be detected in fluid from pregnancies with complications such as preterm labor, preeclampsia, small for gestational age, and premature rupture of membranes (water breaking too soon). It has been theorized that if bacteria are present, then the risk for complications is increased.
However, recent research has indicated that the presence of bacteria in AF may be more normal than once thought, even without complications. A study published in July 2021 found that 37.5% of AF samples from the first and second trimesters showed the presence of bacterial DNA.
Bacterial sequences have also been detected in AF from term deliveries – indicating bacteria did not trigger complications. It is possible that a controlled presence of bacteria in AF could help the fetus develop an immune system prior to birth.
A fetus also commonly has bowel movements in the third trimester (about 13% of live births), and sometimes even in the second trimester, which then enters AF.
The biggest concern regarding meconium-stained AF (brown or green in color) is the increased risk of respiratory distress in the newborn who may have "breathed in" meconium. However, most babies have no issues when this occurs, but some newborns may need respiratory care.
Based on the above, fetal eyelid fusion has an important purpose; it is crucial this fusion occurs before the start of urination or before other contaminants can enter AF. Prior to eyelid fusion (10 weeks of pregnancy), the critical structures within the eye could be damaged as they have no protection.
Once the eyelids fuse, an additional five layers of epithelium develops to cover the eyes, protecting them from AF contaminants once the eyelids separate again around 20 to 24 weeks of pregnancy.
Amount – General
The quantity of amniotic fluid at any time during pregnancy is the product of water exchange between the mother, fetus, and placenta, and can increase or decrease rather quickly at times.
Changes in fluid volume are usually gradual; however – small, temporary shifts can occur within hours due to hydration, the mother’s body position, and activity.
Hydration can potentially and temporarily increase the amount of AF, but more research is needed to determine how much water would need to be consumed and how much extra AF that would amount to; the same research is needed for intravenous hydration for the purpose of increasing fluid. (It is not recommended women drink additional fluids for the purpose of increasing amniotic fluid levels.)
Overall, AF volume normally increases slowly, and is most predictable in the first 20 weeks, when it strongly relates to fetal weight:
By 10 weeks, the average volume is 25 milliliters (ml)
By 12 weeks: 30 to 60 ml
By 16 weeks: 175 ml
From 20 weeks on, there is greater variation of fluid volume:
Keratinization of fetal skin (tougher, more resistant, and no longer porous) begins at 19 to 20 weeks and is usually complete at 27 weeks of pregnancy. Before this time, AF can easily pass in and out of the fetus. Keratinization essentially makes the skin “waterproof”. AF amounts vary between pregnancies in greater degrees after this point.
By 22 weeks: the average volume is 350 mL
By 28 weeks: 800 ml
The total increase of AL is only 5 to 10 ml/day in the third trimester. Women may carry, on average, about 500 to 1,200 ml (0.5 to 1.2 liters) in total between 34 and 38 weeks of pregnancy.
After 38 weeks, fluid volume declines by approximately 125 ml/week, to an average volume of 400 to 800 ml at 40 weeks.
Production of amniotic fluid is mostly created from the mother's blood, as well as fetal urine (a sign of well-being) and fluids from the mouth, nose, trachea, and lungs that exit the fetus.
The fetal urinary system can produce urine at a rate of 500 ml to 1,200 ml/day at term, which is the major source of AF in the second half of pregnancy. Fetuses with kidney problems can have a complete lack of AF.
The fetus swallows this fluid as early as 16 weeks, which helps in the formation of the gastrointestinal tract. Disruption in fetal swallowing is associated with excess AF, known as polyhydramnios.
After keratinization (described above), AF is primarily absorbed through swallowing. It has been estimated that during the final stages of pregnancy, the fetus swallows up to 400 ml per day.
Fetal breathing movements move AF in and out of the lungs, which contributes a small additional amount (of lung fluid) to AF. AF in and out of the respiratory tract is necessary for proper development of these organs.
The rate of contribution of AF from the respiratory tract is 300 to 400 ml a day and has been observed as early 11 weeks of pregnancy. The fetus takes in less than 5 ml per “breath".
Based on the fetus’ activities of swallowing, “breathing”, and urinating, the entire AF volume is recirculated every day.
At birth, the lungs are approximately half-filled with fluid; the first breath replaces the fluid in lungs with air (contractions further help this process).
Disorders of the regulatory process of AF described above can lead to either polyhydramnios (too much AF) or oligohydramnios (too little AF).
AF volume is usually assessed with ultrasound by measuring pockets of fluid to estimate the total volume, observe the structure of the urinary and gastrointestinal tracts, and detect urine in the fetal bladder.
However, precise measurement of AF volume is not currently possible. No single method to assess AF volume has proved to be the most accurate. Most HCPs will use a “2 cm rule”, in which the largest pocket should be more than 2 cm in volume.
Blood tests may also be done to check for disorders that can affect amniotic fluid, such as diabetes or infections.
Polyhydramnios, or hydramnios, is an excess of AF compared with gestational age, generally caused by too much fluid production, too little AF consumption (swallowed), or both. It occurs in approximately 0.3% to 1.6% of all pregnancies.
Further, an HCP may have difficulty feeling the fetus and identifying heart sounds through the abdomen.
Acute polyhydramnios occurs when fluid rapidly accumulates over a period of a few days, and usually occurs in the second trimester (very rarely in the third trimester).
Chronic polyhydramnios, which has a more gradual onset, often starts in the third trimester and is estimated to be about 50 times more frequent than acute.
Polyhydramnios has no identifiable cause in 30% to 60% of cases. Some potential causes that have been identified include:
Diabetes in the mother (most common)
Rh antibodies to the fetus’s blood produced by the mother
A physical birth defect in the fetus, especially a blocked esophagus or defects of the brain and spinal cord
Twin-to-twin transfusion syndrome
Congenital infection (acquired in pregnancy)
With excess AF, the fluid can be removed with a needle through the woman's abdomen if labor begins early or severe problems occur. However, AF is rarely removed unless emergent. If AF is removed, the procedure may need to be performed more than once.
Frequent follow-up visits with an HCP are necessary, as well as possible medication (to reduce amount of fetal urine). Delivery may also be recommended depending on the week of gestation, and the health of the woman and fetus.
Oligohydramnios is a rare but serious condition defined as a decrease in the volume of AF, compared with gestational age, and occurs in about 4% of all pregnancies, mostly after 42 weeks.
As with polyhydramnios, the cause of decreased AF is also not always known. Chronic (gradual) oligohydramnios is likely caused by major fetal anomalies such as obstruction of the urinary tract or kidney problems, resulting in the inability to urinate. Therefore, subsequent excess fluid remains in the fetus (hydrops fetalis).
Decreased AF can also occur from partial or complete placental abruption, chronic high blood pressure, the use of certain medications, prolonged pregnancy, twin-to-twin transfusion syndrome, or if the fetus has died.
Too little fluid for an extended period may cause abnormal or incomplete development of the lungs, intrauterine growth restriction, infection of the amniotic sac or placenta, postmaturity syndrome, lower Apgar scores, delivery complications such as umbilical cord compression or meconium aspiration, deformities in the limbs or face, and fetal death – mostly if the decrease occurs in the second trimester.
It has not been determined whether severe complications result as a direct consequence of the low fluid, or the cause of the low fluid itself.
It is possible that oligohydramnios is only first caught when a woman’s waters (membranes) are purposely ruptured by a HCP during labor, and the amount is smaller than expected.
The safest treatment for oligohydramnios is cesarean delivery if the pregnancy is at least 36 weeks; otherwise, the pregnancy is heavily monitored until the pregnant woman and HCP determine delivery is the best option.
Amnioinfusion, the placement of a catheter through the cervix to administer saline into the amniotic sac, can be used temporarily during labor (and only during labor) to increase fluid until delivery.
Rupture of Membranes ("Water Breaking")
Rupture of the membranes is commonly described as “water breaking.” When the membranes break, the AF within the membranes around the fetus flows out from the vagina, with either a gush or slow leakage.
Premature rupture of the membranes is the leaking of AF from around the fetus at any time before labor starts.
Preterm premature rupture of the membranes is the leaking of AF prior to 37 weeks of pregnancy.
An amniotomy is when these membranes are purposefully “broken” by an HCP during labor, usually to speed up or progress labor.
Read more information on Rupture of Membranes.
Women should attend all prenatal and ultrasound appointments. If problems are identified, early diagnosis and treatment can dramatically improve outcomes for both mother and baby.