I was diagnosed with polyhydramnios on my due date. I went into labor the same day. I didn't have time to find out anything about it, or how to manage it, or what the risks were. I knew nothing other than that I had too much fluid, and couldn't have my baby at the Birth Center because of the cord prolapse risk.
The diagnosis did explain a few things I'd noticed about this pregnancy. My belly got huge in the third trimester, and felt very stretched out, like an over-full balloon. And it never got all bumpy from elbows and knees and butt, it was always round. I only felt major movements, kicks and jabs, and some close-in small movements like fingers scrabbling against my womb. But all other movements weren't registering at all. There was too much cushion of fluid. He also moved around easily in the third trimester, flipping to breech the week before my due date without me even noticing! (He did flip back, though, with some help from my HypnoBirthing instructor.) All of those were indications of more fluid volume than normal. But because even with the extra fluid, the midwives could palpate him easily (my uterus is very stretchy, apparently), they didn't identify fluid volume as an issue.
I wish I'd known a lot of things about it. The backup OB I got at the hospital tried to convince me that I had it because I had undiagnosed
gestational diabetes, and that my baby was SO big I'd be unable to birth him normally. The risk of shoulder distocia from the gestational diabetes was too high, in her opinion. The ultrasound from that morning had measured my son at 10 lbs 10 oz, and she thought I could not birth such a big baby without him getting stuck.
Fortunately, I did know a few things. I knew that late term ultrasounds are notoriously bad at estimating size of baby accurately. I also knew that I'd birthed a large baby before, and had delivered him top-of-the-head first (the biggest dimension of his head) without his head molding at all - he had a perfectly round head, and I hadn't even torn. And that was with a 14.5 inch head, at that! I knew I could birth a 10 lb-plus baby without any trouble. I had faith that I could do it, from previous experience. I also knew that my blood sugar levels had been tested regularly (urine testing, at least) and I never showed even a trace of sugar spilling, even after having something less ideal to eat or drink. I hadn't gained much weight over the pregnancy (about 22 lbs), and most of that had been in the first trimester,
not later, when GD seems to kick in the weight gain. I didn't think the OB was correct about me having undiagnosed gestational diabetes, though I could understand why she had jumped to those conclusions. So I thought for myself, and opted to birth vaginally.
In the end, I was right. My son was only 9 lbs 6 oz. I say 'only' by comparison to what the OB thought. He was big, not huge. He had none of the disproportion normally found with gestational diabetes babies (abnormally large head, broad shoulders, and short neck). I birthed him easily after only a few hours of labor. I didn't tear or need stitches at all. His 15.5 inch head did not mold. The OB even said 'yes, you could birth a 10-lb-plus baby easily' after he slipped smoothly and easily into the world. I knew I didn't have the complications she thought I had, but that was all I knew. So, afterwards, I did some research. Here's what I found:
* About 1 in 200 pregnancies have PH diagnosed in the second trimester, often late in the second or even the third trimester.
* According to several medical school sites, about 60% of the time it is 'idiopathic' - that is, there is no known reason for it, it just is. About 20% of the time it is from a maternal issue, either gestational diabetes or rh incompatibility. And about 20% of the time it is from a fetal issue, including a variety of minor or major defects, like neural tube defects, dwarfism, gastrointestinal obstruction (preventing swallowing or digestion of amniotic fluid), or a benign tumor of the placenta.
In addition to the generalized rates, there are specific rates - if the diagnosis is mild PH, the chance of it being idiopathic is about 80%, where the higher the severity, the more likely that there is an underlying problem of some sort. Severity is determined by measuring the size of the largest pocket of fluid. Even then, the degree of concern may be moderated by how far along you are in the pregnancy - A good neonatologist will take into account your baby's gestational age - a moderate polyhydramnios diagnosis at 40 weeks is not the
same as a moderate PH diagnosis at 28 weeks.
Even in idiopathic cases, there can be other causes at play - they just aren't identified yet. Some mutations such as some genetic forms of muscular dystrophy can cause it, as well, and are included in the idiopathic group. If myotonic dystrophy (the form in question) runs in your family, it is worth being tested for, as it can increase in severity with each generation.
Still, there is a large portion of the incidence of polyhydramnios that is not caused by anything bad that we can tell - the baby just has a larger swimming pool than usual.
There are a few implications of 'normal' polyhydramnios for labor - those being poor positioning of the baby (because it has plenty of room to move around) for labor, macrosomia (large baby - over 4000g, or about 8 3/4 lbs), and primary c-section delivery. The c-section rate can be blamed in part on the poor position issue, and in part on the (anecdotally noted) tendency of polyhydramnios labors to 'stutter' rather than stay active until the water breaks. In some cases, it may be advised for macrosomia, as well - a very large baby, especially one that is not properly positioned, may have more risks in a vaginal birth. Stuttering start to labor may or may not be helped by artificial rupture of membranes.
There are also some general risks with polyhydramnios during labor - both rare, but higher than with normal fluid volume. These are cord prolapse (the cord slipping out with or before the baby, cutting off the blood supply from the placenta), and placental abruption (with sudden and significant 'deflation' of the uterus after the water breaks, the placenta can 'crinkle' away from the uterine wall). However, the one recent study that specifically looked at idiopathic polyhydramnios did not find any implications from these risks - there was no increase in perinatal mortality, preterm birth, low apgar scores, or NICU admissions when compared to normal fluid volume births.
(Idiopathic polyhydramnios and perinatal outcome; Am J Obstet Gynecol 1999 Nov;181(5 Pt 1):1079-82)
So, if you are diagnosed with polyhydramnios, chances are good that everything is fine. The later they notice it, the more likely you have
idiopathic PH. The lower the degree of excess fluid volume, the more likely you have idiopathic PH. But even with a diagnosed reason for it, there isn't necessarily much impact on labor. You will need to be monitored for placental abruption (but fetal monitoring will handle that) and when your water breaks you will need to be checked for cord prolapse, which can be a critical emergency but is still rare. Other than that, polyhydramnios alone should not affect your labor and birth outcomes at all. The other factors that sometimes accompany polyhydramnios, such as macrosomia with gestational diabetes, or certain birth defects, may well affect the birth (such as opting for a c-section in some cases). But the fluid volume alone is not the issue.