Polyhydramnios
A baby in the womb lives surrounded by amniotic fluid. The amount changes throughout pregnancy: it rises until roughly thirty-four weeks and then falls…
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A baby in the womb lives surrounded by amniotic fluid. The amount changes throughout pregnancy: it rises until roughly thirty-four weeks and then falls slightly before birth. Polyhydramnios means there is noticeably more fluid than expected for the stage of pregnancy. It is usually picked up on a routine scan rather than reported as a complaint, and in most cases the pregnancy ends well. But extra fluid is also a clue: sometimes there is a cause behind it that is worth finding early. That is why the line in a scan report is never left without a follow-up.
How much fluid counts as too much
The volume cannot be judged by eye. Neither how a woman feels nor the size of her bump tells you reliably: a bump can be large with a big baby and modest in a slim woman. The person doing the scan measures it, in one of two ways.
- Deepest vertical pocket. The deepest pool of fluid free of the baby and the cord is found and its depth measured. More than eight centimetres means polyhydramnios.
- Amniotic fluid index. The bump is divided into four quadrants, the deepest pocket in each is measured and the four figures are added. More than twenty-four centimetres means polyhydramnios.
The same numbers sort polyhydramnios into mild, moderate and severe. The great majority of cases are mild, and those are exactly the ones that usually stay unexplained and cause no trouble. The bigger the excess, the more likely a specific cause will be found, and the closer the monitoring.
What it feels like
Mild polyhydramnios is usually not felt at all and is found at a routine scan. When there really is a lot of fluid, the womb presses on nearby organs and symptoms appear:
- heaviness and a stretched, bloated feeling in the tummy;
- breathlessness and being unable to take a full breath, particularly lying down;
- heartburn, feeling full quickly, nausea after eating;
- constipation;
- swollen feet, legs and vulva, and varicose veins;
- needing to pass urine often and repeated urine infections;
- a sense that the baby is turning around far too freely.
All of these also happen in ordinary pregnancies, so the diagnosis is never made on symptoms alone. What does point to it at an appointment is something else: the midwife finds it harder to feel the baby's parts through the abdominal wall, and the heartbeat sounds fainter than last time, because there is a lot of fluid between the baby and the device.
A rapid build-up of fluid deserves separate mention. The bump grows over a few days, a stretching pain appears and breathing becomes difficult. This happens more often in twin pregnancies, and it is a reason to be seen promptly rather than to wait for the next booked appointment.
Why the fluid builds up
The causes make more sense once you know where the fluid comes from. In the second half of pregnancy it is mostly the baby's urine, and it disappears because the baby swallows it. The volume rests on the balance between those two flows. Polyhydramnios happens when the baby produces more than usual or swallows less.
- Diabetes in the mother, either gestational or present before pregnancy. High blood sugar in the woman means high blood sugar in the baby, who passes more urine and adds to the fluid. This is the commonest cause that can actually be acted on.
- Twins or triplets. A separate situation is twin-to-twin transfusion syndrome in identical twins sharing a placenta: blood is shared unevenly, so one has too much fluid and the other almost none. It needs care in a centre that deals with it, and sometimes treatment before birth.
- The baby has difficulty swallowing: a blockage in the oesophagus or duodenum, a cleft palate, a lump in the neck, or nerve and muscle conditions that weaken swallowing.
- Anaemia in the baby: rhesus and other blood group incompatibilities, parvovirus infection. The circulation adapts and fluid increases.
- Infections in pregnancy: cytomegalovirus, toxoplasmosis, syphilis.
- Differences in development and genetic syndromes: problems with the heart, kidneys or brain, and chromosomal conditions.
In mild polyhydramnios no cause is found most of the time, and that is a normal result of the work-up rather than a failure of it. The point of looking is to rule out the causes that can be handled in advance.
What else makes a bump large
A bump that measures big for dates does not by itself mean polyhydramnios. It can also be:
- a large baby, including because of that same diabetes;
- twins that have not yet been identified;
- dates that were miscalculated;
- fibroids in the womb;
- body build and weight gain;
- trapped wind or a full bladder at the moment of the examination;
- rarely, fluid in the mother's abdomen or an ovarian mass.
Only a scan can tell these apart, so a tape measure at an appointment is not a diagnosis: it is the reason to arrange the scan.
How it is confirmed
The first test is an ultrasound scan, and it answers two questions at once: it measures the fluid and looks carefully at the baby. The sonographer checks whether the stomach is filled (with a blocked oesophagus it may not be seen), whether there is a double bubble in the abdomen, how the heart, spine, brain and kidneys look, and whether the measurements match the dates. The baby's position is checked too, since with a lot of fluid babies often lie across the womb or bottom down.
What follows depends on what is seen. The usual offers are:
- a glucose tolerance test if diabetes has not been looked for or was tested a while ago;
- a blood test for rhesus antibodies;
- blood tests for infections that matter in pregnancy;
- a Doppler study of the baby's middle cerebral artery if anaemia is suspected;
- a fetal medicine appointment and, in severe polyhydramnios or where differences are seen on the scan, an amniocentesis with genetic testing.
Once polyhydramnios is confirmed, scans are repeated, usually every one to two weeks, to see whether the fluid is still increasing and how the baby is doing.
Monitoring and treatment
Mild polyhydramnios is not treated. It is watched, and most often the volume settles back to normal by the end of pregnancy.
If a cause is found, that is what gets treated: diabetes with diet and, where needed, insulin; an infection according to its type; twin-to-twin transfusion syndrome in a specialist centre.
When there is so much fluid that a woman cannot breathe or sleep and there are still weeks of pregnancy left, some of it is drawn off with a fine needle through the abdominal wall under ultrasound guidance. It gives real relief, but only for a time: the fluid re-accumulates and the procedure is sometimes repeated. It carries risks — starting labour early, introducing infection, placental abruption — so it is done in hospital, and for the woman's comfort rather than to improve a number in a report.
Occasionally indometacin is used, a non-steroidal anti-inflammatory medicine that reduces how much urine the baby produces. It is given as a short course, only in hospital and only up to a certain stage of pregnancy, because it affects an important vessel in the baby's heart. Painkillers from this group — ibuprofen and similar — should not be taken on your own initiative in pregnancy, and least of all in the second half.
When to get help without delay
Contact your midwife or doctor the same day if:
- your bump has grown noticeably over a few days, or a stretching pain has appeared;
- fluid starts to leak or your underwear gets wet for no reason;
- regular contractions start early, or you have a dragging ache in your lower back;
- the baby is moving less often or less strongly than usual, at any stage of any pregnancy;
- you have a temperature, chills or stinging when passing urine.
Call an ambulance — 112 is the single emergency number across much of Europe — if:
- your waters have broken and you feel something firm or a loop of cord in the vagina: until help arrives, kneel with your chest down and hips raised, and do not stand up;
- heavy bleeding starts;
- your tummy becomes hard and painful and the pain does not ease between contractions;
- breathlessness worsens abruptly, you feel faint or you lose consciousness;
- the baby has stopped moving.
Birth and the baby's first days
With moderate or severe polyhydramnios, birth is planned in hospital. Not because labour is bound to be difficult, but because several risks are a little higher than usual and it helps to be able to respond to them at once. The baby lies awkwardly more often, the cord can come down when the waters break, the placenta can separate, contractions are sometimes ineffective, and after the birth an over-stretched womb contracts less well and bleeds more. Even so, a caesarean is not recommended to everyone: plenty of women give birth vaginally.
After the birth a neonatal doctor examines the baby. If the polyhydramnios was never explained, the oesophagus is checked by gently passing a fine tube into the stomach, and the first feeds are watched: how the baby sucks, whether they choke, whether they bring back frothy fluid. These are simple steps, but they are what stops a blockage being missed in the first hours of life, when it is easiest to put right.
Online consultation
The word polyhydramnios in a scan report frightens people more than it deserves, and the question is almost always the same: is this dangerous for the baby. In an online consultation a doctor can go through your reports and blood results, explain what the measurements mean, which tests are worth arranging before your next appointment and which changes need reporting straight away. It is a way of making sense of a report without waiting for a slot. If you are leaking fluid, bleeding, in severe pain or noticing fewer movements, an online appointment is not the right route — that needs an examination in person or an ambulance.
This material is for information only and does not replace medical advice.
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