Amniotic Fluid - 1 - The Basics
While reviewing some of the subjects we have covered over the past 20 months to get some ideas for new posts, it dawned on me that we had never written about one of the most basic topics related to pregnancy that arises as a subject of discussion almost every day in my work for one reason or another - amniotic fluid. In today’s post, we will simply provide the basic background related to amniotic fluid production and clearance and in the posts that follow, we will discuss the significance (causes and effects) of abnormalities of amniotic fluid volume – too much and too little.
Most patients have no clue what amniotic fluid is and just take for granted that it is “just the bag of waters around the baby” that makes it easier to take pretty ultrasound pictures and gets “ruptured” by my doctor to help me labor faster at delivery. Indeed, when there is a clinical reason to discuss amniotic fluid, and I have the opportunity to tell them what it is and where it goes, they are often appalled or simply grossed out. Production and egress of amniotic fluid is a very dynamic process. It isn’t “just there.” Indeed it is estimated that more than 95% of amniotic fluid turns over on daily basis.
Very early in pregnancy, the fluid around the baby is the result of active transport of sodium and chloride across the fetal membranes juxtaposed to maternal tissues. As the salt moves, water follows passively. By the end of first trimester (between 8 and 11 weeks), however, an increasing proportion of amniotic fluid is the result of fetal urine production and later on production and expulsion from the fetal lungs. At the same time the fetal kidneys become the major source of amniotic fluid production, fetal swallowing and the lower gastrointestinal tract become the major means of removing that fluid from around the baby. In other words, the baby pees, then swallows it, then reabsorbs the fluid and passes it back (and forth) across the placenta to the mother. (It is this thought that tends to gross out the parents, so thank goodness the baby usually doesn’t poop much in utero). There are other sources of both fluid production and egress, but for the sake of simplicity, let’s begin our discussion with these basics. The amniotic fluid volume, then, is the sum of the inflows and outflows of the fluid in amniotic space around the baby.
At 16 weeks the amniotic fluid volume (AFV) is about 200 mL; AFV peaks at about 700 to 900 mL at 32-35 weeks in most normal pregnancies and then begins to gradually diminish. By term, the average baby produces about 700 to 900 mL/day of urine and 300 to 400 mL/day of fluid from the lungs. In many pregnancies, there is a dramatic drop in the amniotic fluid volume once the pregnancy gets beyond 42 weeks, corresponding to the time when, even in a ‘normal’ pregnancy, there is an increased risk of ‘unexplained’ fetal death in utero if a woman does not go into spontaneous labor.
In first trimester, the osmolality of amniotic fluid is comparable to that of blood (280 mOsm). However, from early on in pregnancy, the fetal kidneys are capable of retaining salt and other solutes, resulting in the production of urine that is ‘hypotonic’ (60-140 mOsm). This eventually results in a drop in the overall osmolality of the amniotic fluid as the pregnancy progresses. There is a tendency for the baby to lose salt across body surfaces exposed to the amniotic fluid, but usually the baby compensates for this much better than we do when we sit in a bathtub for a little too long and come out looking like a raisin! Fluid exchange between mother and baby occurs freely across the placenta. For this reason, acute changes in maternal hydration (reflected in her plasma osmolality) readily alter fetal hydration. Conditions during pregnancy that can affect maternal plasma volume (low and high), therefore, may be one of the factors that also influences amniotic fluid volume on a short- or long-term basis.
So, that is the basics of amniotic fluid production and overall volume. We actually understand very little about ‘regulation’ of amniotic fluid volume. What we do know, however, is that BOTH too much fluid (polyhydramnios or hydramnios) AND too little fluid (oligohydramnios) are associated with fetal abnormalities and/or poor perinatal outcome, and that’s where we will take this discussion in our next posts….