Amniotic fluid cushions the fetus, regulates temperature, and is essential for fetal lung and musculoskeletal development. Volume is monitored by ultrasound (AFI 5–25 cm normal); deviations — oligohydramnios or polyhydramnios — warrant work-up. Suspected premature rupture of membranes (PROM) is most quickly screened at the bedside with a nitrazine pH test such as AmnioTest™ (FDA 510(k) K914419), which exploits the alkaline pH (7.1–7.3) of amniotic fluid versus acidic vaginal secretions (4.5–6.0).
Key Facts
- Composition — ~98% water plus electrolytes, proteins, lipids, hormones, antibacterial peptides, and shed fetal cells.
- Volume — rises to ~800–1,000 mL by 34–36 weeks, then declines toward term.
- Origin after week 20 — predominantly fetal urine, with smaller contributions from lung secretions.
- Oligohydramnios — AFI < 5 cm; associated with growth restriction, placental insufficiency, or PROM.
- Polyhydramnios — AFI > 25 cm; associated with gestational diabetes, fetal anomalies, or multiple pregnancies.
- PROM bedside test — nitrazine pH; amniotic fluid turns the indicator yellow→blue.
What Amniotic Fluid Is — and Where It Comes From
Amniotic fluid is the clear, slightly yellow liquid that fills the amniotic sac and surrounds the developing fetus from the second week of gestation onward. Early in pregnancy it is derived primarily from maternal plasma diffusing across the amnion; after about week 20, the fetus becomes the dominant source — producing fluid through urination and absorbing it by swallowing in a continuous turnover that recycles the entire volume roughly every 24 hours.
Although water makes up ~98% of the fluid, the remaining ~2% is biochemically active and clinically informative. It contains electrolytes, proteins (including alpha-fetoprotein and IGFBP-1), carbohydrates, lipids, urea, hormones, antibacterial peptides such as lysozyme and defensins, and shed fetal epithelial cells — the same cells used in amniocentesis-based karyotyping and prenatal genetic testing.
Five Functions That Make Amniotic Fluid Essential
1. Mechanical Protection
The fluid acts as a hydraulic shock absorber, dissipating external forces and protecting the fetus from blunt-trauma injury. It also cushions the umbilical cord against compression that would otherwise occur every time the fetus moves against the uterine wall.
2. Temperature Regulation
Amniotic fluid maintains a remarkably stable thermal environment — typically around 37.6°C — insulating the fetus from short-term fluctuations in maternal core temperature and ambient conditions.
3. Fetal Lung Development
This is arguably the most underappreciated role. The fetus inhales and exhales amniotic fluid as part of practice breathing movements. The bidirectional flow of fluid through the airways is a mechanical prerequisite for alveolar branching and surfactant production. Prolonged severe oligohydramnios, particularly in the second trimester, is the principal cause of pulmonary hypoplasia — underdeveloped lungs that may be incompatible with life after birth.
4. Musculoskeletal Development
The buoyancy of the fluid allows the fetus to move freely against gravity, which is necessary for normal muscle tone, joint development, and skeletal symmetry. Severely reduced fluid volumes are associated with limb contractures and positional deformities (the Potter sequence is the classic example).
5. Infection Defense
Amniotic fluid is not sterile of immune activity — it carries antibacterial peptides, complement components, and lactoferrin that inhibit ascending bacterial growth. When the membranes rupture, this barrier is lost and the risk of intra-amniotic infection (chorioamnionitis) rises sharply with the latency interval.
Volume Disorders: Oligohydramnios and Polyhydramnios
Amniotic fluid volume is quantified sonographically using the amniotic fluid index (AFI) — the sum of the deepest vertical pockets in four uterine quadrants. A normal AFI in the third trimester is 5–25 cm. Outside this range, additional work-up is warranted.
| Condition | AFI Threshold | Common Causes |
|---|---|---|
| Oligohydramnios | < 5 cm | Premature rupture of membranes, fetal growth restriction, placental insufficiency, fetal renal anomalies, post-term pregnancy. |
| Polyhydramnios | > 25 cm | Gestational or pre-existing diabetes, multiple gestation, fetal GI obstruction (esophageal/duodenal atresia), neuromuscular disorders impairing swallowing, congenital infection. |
Both conditions can be transient and benign, but both also warrant a targeted anatomic survey and serial monitoring. PROM is the single most common identifiable cause of oligohydramnios in the late second and third trimesters — which is why a fast, reliable bedside test matters.
Premature Rupture of Membranes (PROM)
PROM is rupture of the fetal membranes before the onset of labor; preterm PROM (PPROM) occurs before 37 weeks and complicates roughly 3% of pregnancies but accounts for approximately one-third of all preterm births. Time-to-diagnosis matters: every additional hour of undetected rupture increases the risk of ascending infection, cord prolapse, and placental abruption.
The clinical history — sudden gush of fluid, persistent leakage — is suggestive but unreliable; urinary incontinence, increased physiologic discharge, and loss of the mucus plug all mimic PROM. Confirmation rests on a combination of sterile speculum examination, visualization of fluid pooling in the posterior fornix, and ancillary tests.
The Nitrazine pH Method
Normal vaginal secretions are acidic (pH 4.5–6.0), while amniotic fluid is alkaline (pH 7.1–7.3). A nitrazine indicator — a pH-sensitive dye — turns from yellow to blue on contact with alkaline fluid, providing a result in seconds at the bedside with no instrumentation. The method has been a first-line PROM screen for more than 50 years because it is fast, cheap, and easy to interpret.
AmnioTest™ is a sterile, individually wrapped, dye-impregnated swab cleared by the FDA under 510(k) K914419 for in vitro detection of amniotic fluid leakage. It is used by labor & delivery units, OB clinics, and emergency departments as a rapid adjunct alongside ferning microscopy and clinical assessment. False positives can occur in the presence of blood, semen, antiseptics, or bacterial vaginosis — so nitrazine results are always interpreted in clinical context.
science PROM Screening AmnioTest™ — FDA 510(k) K914419 Nitrazine Swab Sterile, individually wrapped, dye-impregnated swabs. Rapid bedside pH screening for suspected premature rupture of membranes. Direct from Pro-Lab Diagnostics in Georgetown, TX. arrow_forwardPractical Implications for the L&D and OB Clinic
Anything collected during PROM evaluation — speculum-collected fluid for ferning, vaginal swabs for culture, GBS samples — needs to reach the lab intact and uncontaminated. Use OSHA-compliant biohazard transport with absorbent material; TransVelopes specimen bags are the standard at most Texas hospital labs we supply. Glove changes between speculum exam and specimen handling are non-negotiable; size-appropriate, ASTM D6319 nitrile exam gloves are stocked at every L&D bay we audit.
Documentation should record: nitrazine result (color and timing), ferning result (microscopy), AFI on ultrasound if performed, GBS status, and gestational age. Where nitrazine and ferning disagree, repeat testing or biochemical adjuncts (PAMG-1, IGFBP-1) are appropriate per ACOG guidance.
"In suspected PROM, the cost of a missed diagnosis is measured in hours of latent infection. A 5-second bedside pH check is the most under-priced intervention in the entire L&D workflow." — Senior MFM clinician, Texas tertiary center
Frequently Asked Questions
What is amniotic fluid made of?
Amniotic fluid is a clear, slightly yellow liquid composed mostly of water with electrolytes, proteins, carbohydrates, lipids, urea, hormones, antibacterial peptides, and shed fetal cells. After about 20 weeks of gestation, the fluid is largely produced by fetal urine, with smaller contributions from lung secretions and the membranes themselves.
How much amniotic fluid is normal?
Volume increases from a few millilitres in early gestation to roughly 800–1,000 mL at 34–36 weeks, then declines toward term. Clinically, the amniotic fluid index (AFI) is measured by ultrasound; an AFI of 5–25 cm is generally considered normal, with values below 5 cm suggesting oligohydramnios and above 25 cm suggesting polyhydramnios.
What does it mean if amniotic fluid is low or high?
Oligohydramnios (low fluid) may indicate fetal growth restriction, placental insufficiency, fetal renal anomalies, or premature rupture of membranes. Polyhydramnios (excess fluid) can be associated with gestational diabetes, multiple pregnancies, fetal gastrointestinal obstruction, or neuromuscular disorders that impair fetal swallowing.
How is premature rupture of membranes (PROM) diagnosed?
PROM is suspected when a pregnant patient reports a gush or persistent leak of fluid. Confirmation typically uses sterile speculum examination plus a nitrazine pH test: amniotic fluid is alkaline (pH 7.1–7.3) and turns nitrazine indicator from yellow to blue, while normal vaginal secretions remain acidic (pH 4.5–6.0). AmnioTest™ (FDA 510(k) K914419) is a nitrazine-based dye-impregnated swab cleared for this purpose.
Why is the nitrazine pH test still used for PROM?
Because it is fast (results in seconds), inexpensive, easy to interpret at the bedside, and does not require laboratory instrumentation. Nitrazine swabs such as AmnioTest™ remain widely used as a first-line aid in PROM evaluation, often combined with ferning microscopy and clinical assessment.
Can amniotic fluid become infected?
Yes. When the membranes rupture prematurely, ascending bacterial contamination can cause intra-amniotic infection (chorioamnionitis), which is a significant cause of maternal and neonatal morbidity. Rapid identification of PROM and proper specimen transport are therefore essential.
For more information about AmnioTest™ or PROM-evaluation supplies, contact info@pro-lab.us or visit the AmnioTest™ product page.