Amniotic Fluid Index in Normal Pregnancies

What does amniotic fluid do?

Amniotic fluid has several important functions:

•  Provides a cushion to protect the child from trauma (should you fall).
• Keeps the umbilical cord from becoming compressed and reducing the oxygen supply to your baby.
• Keeps the child moving so that bones develop correctly.
• Prevents infection, and helps to keep a regulated temperature in the womb.

The amount of amniotic fluid is measured either by checking the largest pocket of fluid by ultrasound or by measuring the largest amniotic fluid pocket in each of four quadrants and then adding those up. This is called the AFI or Amniotic Fluid Index.

AMNIOTIC FLUID INDEX VALUES (mm) IN NORMAL PREGNANCY
	Amniotic Fluid Index Percentile Values
Week	2.5th	5th	50th		95th	97.5th
16	73	79	121		185	201
17	77	83	127		194	211
18	80	87	133		202	220
19	83	90	137		207	225
20	86	93	141		212	230
21	88	95	143		214	233
22	89	97	145		216	235
23	90	98	146		218	237
24	90	98	147		219	238
25	89	97	147		221	240
26	89	97	147		223	242
27	85	95	146		226	245
28	86	94	146		228	249
29	84	92	145		231	254
30	82	90	145		234	258
31	79	88	144		238	263
32	77	86	144		242	269
33	74	83	143		245	274
34	72	81	142		248	278
35	70	79	140		249	279
36	68	77	138		249	279
37	66	75	135		244	275
38	65	73	132		239	269
39	64	72	127		226	255
40	63	71	123		214	240
41	63	70	116		194	216
42	63	69	110		175	192
Modified from Moore TR, Cayle JE: The amniotic fluid index in normal human pregnancy. Am J Obstet Gynecol1990;162: 1168-1173.

* Normal AFV (amniotic fluid values) in the 2nd half of pregnancy implies at least one functioning fetal kidney.

What is too much or too little amniotic fluid?

The AFI and the single pocket evaluation are 2 ways to evaluate the amniotic fluid.

Polyhydramnios = Too much fluid (usually 25+ cm AFI)

Oligohydramnios = Too little fluid (AFI under 5 cm and/or no pocket over 2 cm)

Because the AFI has too many "false positive" results (too many people diagnosed with too low fluid when in fact the outcome was OK), the single pocket amniotic fluid evaluation has recently been found more reliable than the AFI. Any single pocket that's over 2 cm is considered adequate.

 

What is the treatment for too little or too much amniotic fluid?

Your doctor will perform an ultrasound to confirm the presence of an excess or lack of amniotic fluid to find out where you rate on the amniotic fluid index (AFI). Anything more than 25 cm is too much amniotic fluid, and anything less than 5 cm during the third trimester is too low. 

If you have too much fluid, you will likely have an amniocentesis to test for abnormalities and genetic issues. Testing for gestational diabetes will likely be done, and you will undergo regular ultrasounds and stress tests to monitor the condition of your baby. If you have too little fluid, you will be closely monitored until delivery.

Read More: 
Oligohydramnios: Decreased Amniotic Fluid