The amniotic fluid is the protective liquid contained by the amniotic sac of a gravid amniote. This fluid serves as a cushion for the growing fetus, but also serves to facilitate the exchange of nutrients, water, and biochemical products between mother and fetus.

Colloquially, the amniotic fluid is commonly called water or waters (Latin liquor amnii).

Amniotic fluid is present from the formation of the gestational sac. Amniotic fluid is in the amniotic sac. It is generated from maternal plasma, and passes through the fetal membranes by osmotic and hydrostatic forces. When fetal kidneys begin to function around week 16, fetal urine also contributes to the fluid. In earlier times, it was believed that the amniotic fluid was composed entirely of excreted fetal urine.

The fluid is absorbed through the fetal tissue and skin. After 22 to 25 week of pregnancy, keratinization of an embryo's skin occurs. When this process completes around the 25th week, the fluid is primarily absorbed by the fetal gut for the remainder of gestation.

At first, amniotic fluid is mainly water with electrolytes. By about the 12–14th week the liquid also contains proteins, carbohydrates, lipids and phospholipids, urea, and extracellular matrix (ECM) components including collagens and glycosaminoglycans, including hyaluronic acid and chondroitin sulfate, all of which aid in the growth of the fetus.

The volume of amniotic fluid changes with the growth of the fetus. From the tenth to the 20th week, it increases from 25 to 400 millilitres (0.88 to 14.08 imp fl oz; 0.85 to 13.53 US fl oz) approximately. Approximately in the 10th–11th week, the breathing and swallowing of the fetus slightly decrease the amount of fluid. Neither urination nor swallowing contributes significantly to fluid quantity changes until the 25th week, when keratinization of skin is complete; then the relationship between fluid and fetal growth stops. It reaches a plateau of 800 millilitres (28 imp fl oz; 27 US fl oz) by the 28-week gestational age. The amount of fluid declines to roughly 400 millilitres (14 imp fl oz; 14 US fl oz) at 42 weeks. Some sources indicate about 500 to 1,000 millilitres (18 to 35 imp fl oz; 17 to 34 US fl oz) of amniotic fluid is present at birth.

The forewaters are released when the amnion ruptures. This is commonly known as "water breaking." When this occurs during labour at term, it is known as "spontaneous rupture of membranes". If the rupture precedes labour at term, however, it is referred to as "pre-labour rupture of membranes." Spontaneous rupture of membranes before term is referred to as "premature rupture of membranes." The majority of the hindwaters remain inside the womb until the baby is born. Artificial rupture of membrane (ARM), a manual rupture of the amniotic sac, can also be performed to release the fluid if the amnion has not spontaneously ruptured.

Swallowed amniotic fluid (in later stages of development) creates urine and contributes to the formation of meconium. Amniotic fluid protects the developing fetus by cushioning against blows to the mother's abdomen, allowing for easier fetal movement, and promoting muscular/skeletal development. Amniotic fluid swallowed by the fetus helps in the formation of the gastrointestinal tract. It also protects the fetus from mechanical jerks and shocks. The fetus, which develops within a fluid-filled amniotic sac, relies on the placenta for respiratory gas exchange rather than the lungs. While not involved in fetal oxygenation, fetal breathing movements (FBM) nevertheless have an important role in lung growth and in the development of respiratory muscles and neural regulation. FBM are regulated differently in many respects than postnatal respiration, which results from the unique intrauterine environment. At birth, the transition to continuous postnatal respiration involves a fall in temperature, gaseous distention of the lungs, activation of the Hering-Breuer reflex, and functional connectivity of afferent O2 chemoreceptor activity with respiratory motoneurons and arousal centers.