Amniotic fluid volume: norm by week. Oligohydramnios and polyhydramnios Amount of amniotic fluid by week of pregnancy

Changes in amniotic fluid volume may indicate fetal abnormalities and antenatal complications. Abnormal amniotic fluid volume (AFV) has long been associated with poor perinatal outcome. It is essential to measure AFV during any antenatal fetal examination. The volume of amniotic fluid is characterized as:

• normal;
• oligohydramnios;
• polyhydramnios, or hydroamnion (excess amniotic fluid).

Amniotic fluid volume measurement

The most common objective approach is AFV by amniotic fluid index (AFI) and deepest vertical pouch. AFI is determined by measuring the size of the maximum vertical pocket of amniotic fluid (the sensor is held perpendicular to the mother's abdomen) in each quadrant of the abdomen. A nomogram based on gestational age has been proposed. General classification of amniotic fluid volume using IAF:

• short -<5 см (маловодие);
• the lower limit of normal is 5-9 cm;
• norm - 10-20 cm;
• the upper limit of normal is 20-24 cm;
• high - > more than 24 cm (polyhydramnios).

The measurement of the deepest vertical pocket is also used. AFI is considered normal if this pocket measures 2-8 cm. The use of AFI with a small size of the pregnant uterus is limited when determining AFI up to 24 weeks of pregnancy. Ultrasound criteria for intervals of normal values ​​of the amniotic pouch have been developed only for pregnancy at 11-24 weeks. In general, these semiquantitative measurements of AFW, AFI, and deepest pocket size are equally common. Despite the shortcomings of these techniques, they have advantages over semi-quantitative assessment of AF by the clinician and are convenient for determining dynamics during follow-up using ultrasound. It is believed that the experience of the investigator has little influence on the accuracy of ultrasound assessment of AF. However, for diagnosing AF abnormalities, these measurements are no more accurate than subjective examination by experienced diagnostic ultrasound specialists.

Normal amniotic fluid volume

During pregnancy, AFV always varies slightly individually; in the early stages of pregnancy, AFV increases, peaks at 28-32 weeks, and then begins to decrease from 33 weeks. Despite individual variations, the volume of amniotic fluid ranges from 0.5 to 2 liters. In fairness, it should be noted that this stabilization is the result of the regulation of fetal production and the removal of amniotic fluid during pregnancy. Transport of amniotic fluid into and out of the amnion cavity is regulated primarily by fetal kidney excretion (production) and ingestion (removal). The fetal airway, membranes, and placenta play a minor role in the transport of amniotic fluid. Fetal urine production begins at approximately 9 weeks of gestation, but it is not the main source of amniotic fluid between 14 weeks and 18 weeks of gestation. The last statement is important for understanding AF disturbances in the early and middle of the second trimester. Amniotic fluid performs many functions in the development of the fetus - protection from injury, umbilical cord compression, infection (bacteriostatic properties); it facilitates the development of the lungs, musculoskeletal system and gastrointestinal tract (GIT) of the fetus.

During the process of intrauterine development, the fetus resides in an aquatic environment - amniotic fluid (amniotic fluid), which performs a number of important functions. To determine the condition and development of the fetus, the amount of amniotic fluid is assessed, in particular, the amniotic fluid index (AFI) is used to measure it.

What is IAF and its measurement

The amount of amniotic fluid is assessed in two ways:

Subjective.
A sonographer (ultrasound specialist) carefully examines the amount of amniotic fluid in transverse and longitudinal scans and identifies polyhydramnios (an increase in amniotic fluid between the fetus and the anterior abdominal wall) or oligohydramnios (accordingly, the volume of water is reduced along with spaces free from echo structures).

Objective.
This method is the determination of the amniotic fluid index. To do this, the ultrasound specialist divides the uterine cavity into 4 quadrants by drawing perpendicular lines. The transverse line runs horizontally at the level of the navel, and the vertical line runs along the linea alba of the abdomen. In each “compartment”, a vertical pocket is determined and measured, that is, the deepest pocket without echo structures. By summing the 4 values, which are expressed in centimeters, the IAF is obtained.

Oligohydramnios is established when the depth of the largest pocket of amniotic fluid is less than 2 cm, and polyhydramnios when the depth of the largest pocket is above 8 cm.

Amniotic fluid norms by week

The amniotic fluid index depends on the stage of pregnancy, and starting from 16 weeks, its value gradually increases, reaching a peak at 32 weeks, and then AFI values ​​decrease.

Amniotic fluid index norms:

• 16 weeks – 73-201mm (average 121mm);
• 17 weeks – 77-211mm (average 127mm);
• 18 weeks – 80-220mm (average 133mm);
• 19 weeks – 83-230mm (average 137mm);
• 20 weeks – 86-230mm (average 141mm);
• 21 weeks – 88-233mm (average 143mm);
• 22 weeks – 89-235mm (average 145mm);
• 23 weeks – 90-237mm (average 146mm);
• 24 weeks – 90-238mm (average 147mm);
• 25 weeks – 89-240mm (average 147mm);
• 26 weeks – 89-242mm (average 147mm);
• 27 weeks – 85-245mm (average 156mm);
• 28 weeks – 86-249mm (average 146mm);
• 29 weeks – 84-254mm (average 145mm);
• 30 weeks – 82-258mm (average 145mm);
• 31 weeks – 79-263mm (average 144mm);
• 32 weeks – 77-269mm (average 144mm);
• 33 weeks – 74-274mm (average 143mm);
• 34 weeks – 72-278mm (average 142mm);
• 35 weeks – 70-279mm (average 140mm);
• 36 weeks – 68-279mm (average 138mm);
• 37 weeks – 66-275mm (average 135mm);
• 38 weeks – 65-269mm (average 132mm);
• 39 weeks – 64-255mm (average 127mm);
• 40 weeks – 63-240mm (average 123mm);
• 41 weeks – 63-216mm (average 116mm);
• 42 weeks – 63-192mm (average 110mm).

Reasons for deviations from the norm

Deviations from the norm of AFI, up or down, make it possible to diagnose polyhydramnios and oligohydramnios.

Polyhydramnios

Polyhydramnios occurs in 1-3% of cases of the total number of births (data vary significantly among different authors). This pathology is caused by 3 groups of factors:

1. From the woman's side

• immunization based on Rh factor and blood group;
• diabetes;
• various infectious and inflammatory processes;

2. From the placenta

• Chorioangioma (benign tumor of the fetal membrane, the outcome of pregnancy depends on its size; if detected, regular monitoring is recommended);
• according to ultrasound, “placenta surrounded by a cushion” (edema);

3. From the fetus

• multiple pregnancy;
• congenital anomalies of fetal development (with polyhydramnios, it occurs in 20-30% of cases);
• chromosomal pathology and hereditary diseases;

Low water

The share of oligohydramnios is 0.3-5.5% and the causes of its occurrence include:

• intrauterine anomalies of fetal development (malformations of the genitourinary system);
• fetal pathology (developmental delay, intrauterine infection, chromosomal diseases);
• maternal diseases (preeclampsia, pathology of the cardiovascular system, kidney disease, infectious and inflammatory processes);
• pathology of the placenta (fetoplacental insufficiency, placental defects, heart attacks);
• post-maturity;
• placental abruption;
• intrauterine fetal death.

Limit values ​​of sizes The largest vertical pockets for defining oligohydramnios are somewhat arbitrary and vary in their positive predictive value and sensitivity. Based on our clinical experience with more than 5,500 high-risk patients, a maximum water pocket cut-off value of 2 cm appears to be the most appropriate and recommended for use, as the greatest increase in adverse perinatal outcomes is observed from around this level.

Results more 145,000 examinations with measurements of amniotic fluid volume in more than 75,000 high-risk pregnant women, including more than 10,000 patients with suspected IUGR, allowed us to discover certain clinical rules.

1. Availability of normal volume amniotic fluid does not completely exclude the diagnosis of any of the etiological types of IUGR, but the likelihood of this pathology in such situations is sharply reduced.

2. U fetus with signs of growth retardation according to fetometry, with the confirmed presence of kidneys and their functioning, as well as with intact membranes, oligohydramnios (with a maximum “pocket” of less than 2 cm) is a sign that with high statistical power indicates IUGR due to placental insufficiency. Identification of this sign is important for developing optimal management tactics, since it implies delivery of a fetus without malformations, for which extrauterine survival is at least the method of choice.

In our experience, practical rule that a fetus with oligohydramnios always suffers from retarded development, unless proven otherwise, is reliable and eminently useful.

3. It is important to distinguish between secondary oligohydramnios, caused by placental insufficiency, which can also occur with chromosomal disorders, especially with trisomies 13 and 18.

4. In all cases, inalienable part of an echographic examination remains to confirm the presence of kidneys and their functioning in the fetus, since renal agenesis, which is a lethal anomaly, in classical cases manifests itself in the second half of pregnancy in the form of severe IUGR and oligohydramnios.

Fetal examination with non-functioning urinary system due to agenesis or primary dysgenesis of the renal parenchyma presents significant difficulties. In such situations, after 20 weeks of pregnancy, an echographic examination almost always reveals pronounced IUGR and oligohydramnios, which worsens the conditions for imaging and makes it extremely difficult to diagnose fetal kidney pathology. Differential diagnosis of such conditions from true severe IUGR is of critical clinical importance, since the tactics for managing pregnancy in patients will be radically different. Detection of fetal urine in the bladder and changes in its volume over time (hourly assessment) will indicate a diagnosis of IUGR, provided that the integrity of the membranes is established.

Such situations It was proposed to prescribe furosemide to the mother, which was expected to have a diuretic effect on the fetus by penetrating the placenta. Despite the attractiveness of this theoretical assumption, it has not been confirmed, since transplacental penetration of furosemide was not detected in sheep fetuses in experiments. According to some researchers, the administration of diuretics to pregnant women with fetal growth retardation and established functioning of the urinary system did not stimulate diuresis in antenatal conditions. The discovery of a pronounced decrease in the ratio of chest circumference to abdominal circumference most likely indicates the presence of pulmonary hypoplasia, which is almost always found in fetuses with renal agenesis/dysgenesis.

There are descriptions of experimental techniques, which could help resolve this kind of differential diagnostic difficulties, for example, by adding saline solution to the amniotic cavity to improve visualization of the area where the kidneys are located in the abdominal cavity of the fetus, as well as intramuscular and intravenous administration of diuretics to the fetus or intravenous administration of other infusion solutions that will help improve glomerular filtration.