The amniotic fluid index (AFI) is the sum of the deepest pocket of fluid in each of four quarters of the womb, measured in centimetres on an ultrasound scan. A normal AFI changes with the week of pregnancy: in the reference study scanners still use, the median rose from 12.1 cm at 16 weeks to about 14.7 cm between 24 and 26 weeks, then fell to 12.3 cm at 40 weeks.
This page has the AFI chart for every week from 16 to 42, a calculator for the four numbers on your scan report, and what a low or high result does and does not mean. It also explains why many units now prefer a different measurement, the deepest vertical pocket.
The short answer: an AFI between 5 and 24 cm is usually reported as normal. At 32 weeks the median is 14.4 cm and the middle 90% of normal pregnancies measure 8.6–24.2 cm. An AFI of 5 cm or less is called oligohydramnios (low fluid); 24 cm or more is polyhydramnios (too much fluid).
What is a normal amniotic fluid index by week?
The most widely used week-by-week reference comes from Moore and Cayle, who measured the AFI in 791 normal pregnancies and found that each week had its own range. The table gives the median for each week, the range that held the middle 90% of pregnancies (5th to 95th percentile) and the wider middle 95%.
| Week | Median, cm | 5th–95th percentile, cm | 2.5th–97.5th percentile, cm |
|---|---|---|---|
| Week 16 | 12.1 | 7.9 – 18.5 | 7.3 – 20.1 |
| Week 17 | 12.7 | 8.3 – 19.4 | 7.7 – 21.1 |
| Week 18 | 13.3 | 8.7 – 20.2 | 8 – 22 |
| Week 19 | 13.7 | 9 – 20.7 | 8.3 – 22.5 |
| Week 20 | 14.1 | 9.3 – 21.2 | 8.6 – 23 |
| Week 21 | 14.3 | 9.5 – 21.4 | 8.8 – 23.3 |
| Week 22 | 14.5 | 9.7 – 21.6 | 8.9 – 23.5 |
| Week 23 | 14.6 | 9.8 – 21.8 | 9 – 23.7 |
| Week 24 | 14.7 | 9.8 – 21.9 | 9 – 23.8 |
| Week 25 | 14.7 | 9.7 – 22.1 | 8.9 – 24 |
| Week 26 | 14.7 | 9.7 – 22.3 | 8.9 – 24.2 |
| Week 27 | 14.6 | 9.5 – 22.6 | 8.5 – 24.5 |
| Week 28 | 14.6 | 9.4 – 22.8 | 8.6 – 24.9 |
| Week 29 | 14.5 | 9.2 – 23.1 | 8.4 – 25.4 |
| Week 30 | 14.5 | 9 – 23.4 | 8.2 – 25.8 |
| Week 31 | 14.4 | 8.8 – 23.8 | 7.9 – 26.3 |
| Week 32 | 14.4 | 8.6 – 24.2 | 7.7 – 26.9 |
| Week 33 | 14.3 | 8.3 – 24.5 | 7.4 – 27.4 |
| Week 34 | 14.2 | 8.1 – 24.8 | 7.2 – 27.8 |
| Week 35 | 14 | 7.9 – 24.9 | 7 – 27.9 |
| Week 36 | 13.8 | 7.7 – 24.9 | 6.8 – 27.9 |
| Week 37 | 13.5 | 7.5 – 24.4 | 6.6 – 27.5 |
| Week 38 | 13.2 | 7.3 – 23.9 | 6.5 – 26.9 |
| Week 39 | 12.7 | 7.2 – 22.6 | 6.4 – 25.5 |
| Week 40 | 12.3 | 7.1 – 21.4 | 6.3 – 24 |
| Week 41 | 11.6 | 7 – 19.4 | 6.3 – 21.6 |
| Week 42 | 11 | 6.9 – 17.5 | 6.3 – 19.2 |
Source: Moore and Cayle, American Journal of Obstetrics and Gynecology 1990, 791 normal pregnancies; table as reproduced in GLOWM and by Callen (UCSF), converted from millimetres. Some units use their own charts.
Notice that even the 2.5th percentile never falls below 6.3 cm. A result can sit below the 5th percentile for its week and still be above the 5 cm line that defines low fluid. That is why a report may say "AFI low-normal" or ask for a repeat scan without using the word oligohydramnios.
AFI calculator
Enter the four pocket measurements from your scan report, or the total AFI if that is all it shows, and the week of the scan. The calculator adds them up and places the result against the week's range and the usual cut-offs.
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Whatever the number: fluid leaking from your vagina or a baby moving less than usual needs a call to your maternity unit straight away.
How the AFI is measured
The sonographer divides your bump into four quarters, using the belly button as the centre, and measures the deepest clear pocket of fluid in each one, straight up and down. The four numbers added together are the AFI, a method described by Phelan and colleagues in 1987. To count, a pocket has to be at least 1 cm wide and free of the baby's limbs and the umbilical cord.
The other common measurement is the deepest vertical pocket (DVP, also called the maximum vertical pocket or single deepest pocket): the depth of the single largest pocket anywhere in the womb. A DVP between 2 and 8 cm is normal.
AFI or deepest pocket: which is better?
For spotting low fluid, the evidence favours the deepest pocket. A Cochrane review of 4 trials with 3,125 women found that using the AFI diagnosed low fluid more than twice as often and led to about twice as many inductions of labour, with no better outcomes for the babies. A later German trial of 1,052 women at term found the same. ISUOG's 2024 guideline therefore prefers the deepest pocket for diagnosing low fluid, while the AFI may still be the better tool for judging too much fluid.
So if your report gives both numbers and they seem to disagree, that is expected: they measure different things and have different cut-offs.
Low amniotic fluid (oligohydramnios)
Low fluid is usually defined as an AFI of 5 cm or less, or a deepest pocket under 2 cm. It often causes no symptoms at all. Known causes include the waters leaking before labour, a placenta that is not working as well as it should (for example with high blood pressure), a pregnancy past 42 weeks, some medicines such as NSAIDs and ACE inhibitors, problems with the baby's kidneys or growth, and in many cases nothing that can be found.
What follows depends on the week and the cause. Usually it means more scans to check the baby's growth and fluid, heart-rate monitoring and, if low fluid persists late in pregnancy, a plan to deliver a little before the due date.
High amniotic fluid (polyhydramnios)
The Society for Maternal-Fetal Medicine (SMFM) defines too much fluid as an AFI of 24 cm or more, or a deepest pocket of 8 cm or more. It grades it mild at 24 to 29.9 cm, moderate at 30 to 34.9 cm and severe at 35 cm or more, and notes that about two in three cases are mild. ISUOG draws the line slightly higher, above 25 cm.
SMFM advises looking for a cause whenever too much fluid is found. The two most common are diabetes in the mother and conditions in the baby that affect swallowing or the kidneys; twins, infections and rhesus antibodies are rarer causes. Most cases, though, have no cause at all: SMFM puts these "idiopathic" cases at about 60–70%, most of them mild. For mild idiopathic polyhydramnios it recommends letting labour start on its own at term, with no induction before 39 weeks unless there is another reason.
Many women with mild polyhydramnios notice nothing. With a lot of fluid, breathlessness or painful contractions before the due date can happen, and a bump that suddenly grows needs checking the same day.
When to call your maternity unit the same day
Call your midwife or maternity unit straight away, even at night, if: fluid leaks or gushes from your vagina (your waters may have broken), your baby is moving less than usual or the pattern has changed, you have vaginal bleeding, or your bump suddenly gets bigger, with tummy pain or breathlessness. Do not wait for your next scan. Call 999 or 911, or your local emergency number, for heavy bleeding or if you collapse.
An AFI figure on a report is never on this list by itself. A result outside the week's range is something to talk through with the person who scanned you or at your next appointment.
Frequently Asked Questions
What is a normal amniotic fluid index at 32 weeks?
In Moore and Cayle's reference study, the median AFI at 32 weeks was 14.4 cm, and the middle 90% of normal pregnancies measured between 8.6 and 24.2 cm. Most units treat anything from just over 5 cm to under 24 cm as normal, so a value outside the week's range but inside those limits is usually watched rather than acted on.
What is a normal AFI at 36 weeks?
At 36 weeks the median was 13.8 cm, with the middle 90% between 7.7 and 24.9 cm. The fluid volume is at its widest spread around this time and starts to fall after it, which is expected.
Does amniotic fluid decrease at the end of pregnancy?
Yes. The median AFI stays around 14.7 cm through the middle of pregnancy and falls after about 35 weeks, to 12.3 cm at 40 weeks and 11 cm at 42 weeks in Moore and Cayle's data. That fall is one reason pregnancies past their due date are monitored more closely.
What AFI is too low?
An AFI of 5 cm or less is the usual definition of oligohydramnios (low fluid), or a deepest single pocket under 2 cm. ISUOG and a Cochrane review favour the deepest pocket, because using the AFI labels more women as having low fluid and leads to more inductions without better outcomes for babies.
What AFI is too high?
SMFM defines polyhydramnios as an AFI of 24 cm or more, or a deepest pocket of 8 cm or more. It is graded mild up to 29.9 cm, moderate from 30 cm and severe from 35 cm. ISUOG draws the line slightly higher, above 25 cm, so your unit's cut-off may differ by a centimetre.
Can drinking more water increase amniotic fluid?
Staying well hydrated matters in pregnancy, but drinking extra water is not a recognised treatment for low fluid, and none of the guidelines on this page recommend it as one. If your fluid is low, the follow-up depends on the cause, the baby's growth and how far along you are; our guide to how much water to drink in pregnancy covers daily needs.
Can ibuprofen affect amniotic fluid?
Yes. The US FDA warns that NSAIDs such as ibuprofen, taken from 20 weeks of pregnancy, can lower the amount of amniotic fluid, which is why they are avoided from then on unless a doctor advises them. Paracetamol is the usual choice for pain or fever in pregnancy.
Amniotic fluid index by week — at a glance
The amniotic fluid index adds up the deepest pocket of fluid in each quarter of the womb. In Moore and Cayle's reference data the median AFI rises from 12.1 cm at 16 weeks to about 14.7 cm in mid-pregnancy and falls to 12.3 cm at 40 weeks, and each week has its own range. Most units call 5 to 24 cm normal: 5 cm or less is oligohydramnios, 24 cm or more polyhydramnios, graded mild, moderate or severe. For low fluid, the deepest vertical pocket (under 2 cm) is now often preferred, because the AFI over-diagnoses it and leads to more inductions without better outcomes. Leaking fluid, fewer movements, bleeding or a suddenly bigger bump need a call to your maternity unit straight away.
Inside Baby Novum: the ultrasound tracker keeps each scan with its date, gestational week and a photo of the report, so the fluid measurement from this scan sits next to the last one when you talk it through. One record is free; Premium keeps every scan with charts.
Read next
Sources
- Moore & Cayle, American Journal of Obstetrics and Gynecology 1990 — The amniotic fluid index in normal human pregnancy
- GLOWM, Fischer RL — Amniotic fluid: physiology and assessment (Table 4, AFI values in normal pregnancy)
- Phelan et al., Journal of Reproductive Medicine 1987 — Amniotic fluid volume assessment with the four-quadrant technique
- Society for Maternal-Fetal Medicine Consult Series #46 — Evaluation and management of polyhydramnios (2018, reaffirmed 2024)
- ISUOG Practice Guidelines (2024) — Performance of third-trimester obstetric ultrasound scan
- Nabhan & Abdelmoula, Cochrane Database of Systematic Reviews 2008 — Amniotic fluid index versus single deepest vertical pocket
- Kehl et al., Ultrasound in Obstetrics & Gynecology 2016 — Single deepest vertical pocket or amniotic fluid index (SAFE trial)
- MSD Manual Consumer Version — Problems with amniotic fluid
- MSD Manual Professional Version — Oligohydramnios
- NHS — Polyhydramnios (too much amniotic fluid)
- NHS — Your baby's movements
- US FDA — Avoid NSAIDs in pregnancy at 20 weeks or later because they can result in low amniotic fluid
Baby Novum is an informational and tracking tool, not a medical device, and does not provide medical diagnosis or treatment. Always consult your doctor or midwife about your pregnancy.