What is amniotic fluid?
Amniotic fluid is produced as soon as the gestational sac is formed and, along with the developing embryo (and later fetus), is contained within the amniotic sac. The amniotic sac is a thin but tough pair of membranes that is contained within the gestational sac and is also known as the “bag of waters”.
Amniotic fluid is initially generated solely by the mother’s plasma but when the fetal kidneys begin to function at around week 16 fetal urine also contributes to the fluid. Although initially absorbed through the fetal tissue, once the fetus’ skin is keratinized the amniotic fluid is absorbed by the fetal gut when the fetus ingests the liquid.1
In the beginning amniotic fluid mainly comprises water with electrolytes but by around week 12 to 14 it also contains lipids, carbohydrates, urea, protein, and phospholipids – all of which are vital in helping the fetus to develop.
The volume of amniotic fluid increases as the fetus grows – increasing from from 25 ml to 400 ml between the 10th and 20th weeks of pregnancy The volume reaches a plateau by 28 weeks of pregnancy of 800 ml but then declines to around 400 ml by 42 weeks of pregnancy.1
If the amniotic sac ruptures prematurely or if the amniotic sac ruptures at term but labor is not imminent there is an increased risk of infection as bacteria are now able to enter the amniotic sac. In addition it can also increase the risk of cord compression. If either of these situations occurs you should get yourself to the hospital as soon as possible in order to undergo appropriate treatment.2,3
What is the function of amniotic fluid?
Amniotic fluid protects the developing fetus by providing a barrier between the fetus and the mother’s abdomen protecting it from blows and mechanical shocks. It also allows for easier fetal movement and promotes development of the muscles and skeleton of the fetus. Amniotic fluid also creates urine when swallowed and contributes to the formation of meconium, the earliest stool of your baby.1,4
It used to be thought that the fetus inhaled and exhaled amniotic fluid but actually there is no inward flow of amniotic fluid into the airway. Instead lung development occurs due to the production of fetal lung fluid which expands the lungs.1
Why amniotic fluid levels during pregnancy matter
Although a low level of amniotic fluid (also called oligohydramnios) often still results in a healthy pregnancy and baby it can cause or indicate issues for both the baby and the mother. Babies surrounded by too little amniotic fluid can develop clubbed feet or hands, contractures of the limbs, or can develop a potentially fatal condition called hypoplastic lungs whereby the lungs are small and underdeveloped. This means that when the baby is born he or she can die due to lack of oxygen.
Oligohydramnios can be caused by infection, kidney dysfunction or malformation (since much of the late amniotic fluid volume is urine), procedures such as chorionic villus sampling and preterm premature rupture of membranes. It can sometimes be treated with bed rest, antibacterials, steroids, oral and intravenous hydration, and amnioinfusion
At the other end of the spectrum there can be an excessive amount of amniotic fluid in the amniotic sac during pregnancy – this is called polyhydramnios. In this situation the fetus can be at risk of a number of problems such as cord prolapse placental abruption premature birth and perinatal death.6,7
There is also an often fatal condition associated with the amniotic fluid called amniotic fluid embolism. In this condition amniotic fluid enters the bloodstream of the mother triggering a serious reaction which then results in collapse of the lungs and heart and excessive bleeding.8 Luckily this condition is extremely rare!
How are amniotic fluid levels measured?
During each prenatal visit, your healthcare provider should measure your fundal height with a tape measure. The fundal height is measured from the top of your uterus to the top of your pubic bone and gives a good estimation of the size of your uterus. It is important that the fundal height be measured and properly recorded to track proper fetal growth and the increasing development of amniotic fluid. You should also be undergoing regular ultrasounds by your healthcare provider which will give an additional indication of proper fetal growth and amniotic fluid development.1