What is being measured

Amniotic fluid volume cannot be measured directly in practice, so ultrasound estimates it. Cochrane reviewers describe the two methods used: the amniotic fluid index (AFI), where "the operator divides the uterine cavity into four quadrants" and sums the deepest vertical pocket in each, and the single deepest vertical pocket (SDVP), which uses the largest single pool.

On the thresholds, the review states: "According to these two methods, an AFI of 5 cm or less, or the absence of a pocket measuring 2 x 1 cm is indicative of decreased AFV."

The same review is unusually honest about how arbitrary this is: "Different arbitrary cut-off values for identifying oligohydramnios have been estimated to be 5 cm or 8 cm", and it notes that methods of assessing amniotic fluid "perform best when identifying normal volumes, but are poor when identifying an abnormal volume". It also records that in one case-control study, "72% of women with an AFI of 5 cm or less still had a SDVP measurement of greater than 2 cm".

The method changes the diagnosis

This is not a technicality; it is the single most useful thing to know about this diagnosis. The Cochrane meta-analysis of four trials in 3,125 women found that when the amniotic fluid index was used rather than the single deepest pocket:

"significantly more cases of oligohydramnios were diagnosed (risk ratio (RR, random) 2.33, 95% CI 1.67 to 3.24)"
"more women had inductions of labor (RR (fixed) 2.10, 95% CI 1.60 to 2.76)"
and more had "caesarean delivery for fetal distress (RR (fixed) 1.45, 95% CI 1.07 to 1.97)"

with, crucially, "no evidence that one method is superior to the other in the prevention of poor peripartum outcomes, including: admission to a neonatal intensive care unit; an umbilical artery pH of less than 7.1; the presence of meconium; an Apgar score of less than 7 at five minutes; or caesarean delivery."

The reviewers' conclusion: "The single deepest vertical pocket measurement in the assessment of amniotic fluid volume during fetal surveillance seems a better choice since the use of the amniotic fluid index increases the rate of diagnosis of oligohydramnios and the rate of induction of labor without improvement in peripartum outcomes."

Twice as many diagnoses, twice as many inductions, no measurable benefit. That is a reasonable thing to ask about: which method was used for my scan?

Why fluid falls

Amniotic fluid in the second half of pregnancy is largely fetal urine, cleared largely by the baby swallowing it and by absorption across the membranes. Cochrane summarises the causes of a reduced volume as "fetal anomalies, intrauterine growth restriction, prolonged (post-term) pregnancies, and pre-eclampsia".

Green-top Guideline No. 31 puts the growth-restriction link in clinical terms: growth restricted babies "may manifest evidence of fetal compromise (abnormal Doppler studies, reduced liquor volume)". Low fluid is often a sign that the placenta is underperforming rather than a problem in its own right, which is why the investigation usually widens rather than stopping at the fluid.

The first question: have your waters broken?

Ruptured membranes are the commonest reversible explanation, and they are not always obvious. NICE guideline NG25 sets out how this is checked: "In a woman reporting symptoms suggestive of P-PROM, offer a speculum examination to look for pooling of amniotic fluid", and if pooling is seen, "do not perform any diagnostic test but offer care consistent with the woman having P-PROM". If no pooling is seen, an insulin-like growth factor binding protein-1 test or placental alpha-microglobulin-1 test of vaginal fluid is performed. NICE specifically says not to use nitrazine to diagnose P-PROM.

If preterm prelabour rupture of membranes is confirmed, NICE recommendation 1.4.1 asks for "oral erythromycin 250 mg 4 times a day for a maximum of 10 days or until the woman is in established labour (whichever is sooner)", and specifically says not to offer co-amoxiclav. Intrauterine infection is diagnosed on "a combination of clinical assessment and tests (C-reactive protein, white blood cell count and measurement of fetal heart rate using cardiotocography)" rather than on any one of them alone.

On timing, NG207 says not to induce labour before 34+0 weeks for P-PROM "unless there are additional obstetric indications (for example, infection or fetal compromise)", offering expectant management until 37+0 weeks; between 34+0 and 37+0 weeks the options are discussed with you.

What monitoring looks like

Amniotic fluid volume is one of the five components of the biophysical profile. ACOG describes it as evaluating fetal heart rate, breathing movements, body movements, muscle tone and "amount of amniotic fluid", each scored 0 or 2 for a total out of 10, where "a score of 8-10 is reassuring" and "a score of 6 is equivocal".

Alongside that you would expect growth measurement and umbilical artery Doppler, because those are the tests that distinguish a well-grown baby with a low pocket from a baby whose placenta is failing. Green-top Guideline No. 63 makes the link explicit in labour: women with minor antepartum haemorrhage "with evidence of placental insufficiency (such as fetal growth restriction or oligohydramnios) should be recommended to undergo continuous electronic fetal monitoring".

What I could not find a UK threshold for

Worth saying plainly, because plenty of pages will quote you a number as though it were settled. There is no NICE or RCOG recommendation that names a gestation at which birth should be planned for isolated, otherwise uncomplicated oligohydramnios. The Cochrane evidence above explains part of why: the diagnosis itself is method-dependent and the interventions it triggers have not been shown to improve outcomes.

What that means in practice is that timing of birth in isolated oligohydramnios is an individual decision made on the whole picture - gestation, growth, Dopplers, CTG, whether membranes have ruptured and whether anything else is going on - rather than on the fluid measurement alone. If you are offered a date, ask what else it is based on.

Questions worth asking

Was the AFI or the single deepest pocket used, and what was the actual figure? Is my baby's growth on track, and what is the umbilical artery Doppler showing? Have ruptured membranes been excluded, and how? Is anything else - blood pressure, growth, movements - contributing to the plan? And what would change the plan between now and the next scan?

Those questions move the conversation from a single number to the thing the number is standing in for, which is how well the placenta is working.

Oligohydramnios