Pregnancy · Fetal surveillance
AFI / SDP Interpreter
Interpret amniotic fluid measurement using AFI (sum of 4 quadrant pockets) or SDP (single deepest pocket). SMFM and ACOG now recommend SDP for routine surveillance — fewer false positives and unnecessary interventions than AFI (Nabhan 2008 Cochrane).
Last reviewed 25 May 2026
Amniotic fluid — AFI or SDP
Amniotic Fluid Index / Single Deepest Pocket
Method
cm
wk
Enter the measurement and gestational age to interpret.
Educational tool only — not medical advice. SDP ≤ 2 cm is the SMFM and ACOG-preferred measure for oligohydramnios in routine surveillance (fewer false positives than AFI ≤ 5 cm — Nabhan 2008 Cochrane). For polyhydramnios, AFI is still commonly used. Amniotic fluid trends matter as much as a single value.
What does this mean?
Amniotic fluid is mostly fetal urine after about 20 weeks, so fluid volume is a real-time indicator of fetal kidney function, placental perfusion, and membrane integrity. Oligohydramnios (too little) is a red flag for membrane rupture, placental insufficiency / IUGR, post-term pregnancy, or congenital renal/urinary anomalies — at term, most centres recommend induction. Polyhydramnios (too much) commonly reflects maternal diabetes, multiple gestation, or fetal swallowing/ neuromuscular problems; severe cases raise the risk of preterm labour, malpresentation, and PPROM. The Cochrane meta- analysis (Nabhan 2008) found that SDP picks up the same poor outcomes as AFI while reducing unnecessary interventions, so SMFM/ACOG/ISUOG now prefer SDP ≤ 2 cm for oligohydramnios surveillance. Trends over successive scans matter as much as any single value.
Introduction
Amniotic fluid volume is one of the parameters tracked in fetal surveillance. It reflects the balance of fetal urine production, fetal swallowing, and placental membrane transport. Significant abnormalities (oligohydramnios or polyhydramnios) signal a need for further investigation.
The two measurement methods
AFI (Phelan 1987)
Sum of the deepest vertical pocket in each of 4 uterine quadrants. Range 5-25 cm at term; ≤ 5 cm oligo, > 25 cm poly. Easier in early pregnancy when the fetus is small.
SDP (Chamberlain 1984)
Largest single vertical pocket free of cord or fetal parts. Normal 2-8 cm. < 2 cm oligo, > 8 cm poly. SMFM and ACOG prefer for routine surveillance.
Why SDP is preferred
Nabhan 2008 Cochrane review of 5 trials and 3,226 high-risk pregnancies found that using AFI ≤ 5 cm as the oligohydramnios threshold led to:
- More inductions of labour (RR 2.10).
- More caesarean sections for fetal distress (RR 1.51).
- NO improvement in perinatal outcomes.
SDP ≤ 2 cm catches the genuine oligohydramnios cases without flagging the false positives. SMFM Consult on Antenatal Fetal Surveillance now recommends SDP; ACOG Committee Opinion 828 agrees for routine indications.
Oligohydramnios — causes and management
Causes
- Ruptured membranes (most common — fluid loss externally).
- Fetal growth restriction with placental insufficiency.
- Post-term pregnancy.
- Fetal renal anomaly (agenesis, polycystic disease, obstruction).
- Maternal medications (NSAIDs, ACE inhibitors).
- Maternal dehydration (transient).
Workup
- Speculum / pad check for rupture of membranes.
- Fetal growth and Doppler ultrasound.
- Detailed anatomy scan if not already done.
- Medication review.
Management
- At term: induction usually appropriate.
- Preterm: balance preterm risk against intrauterine risks; surveillance with serial scans.
- Maternal IV / oral hydration may transiently improve AFI.
Polyhydramnios — causes and management
Causes
- Maternal diabetes (esp. poorly controlled).
- Fetal swallowing impairment (CNS anomaly, oesophageal atresia, neuromuscular).
- Fetal hydrops (anaemia, infection, cardiac).
- Monochorionic twin TTTS.
- Idiopathic (~50 % of cases).
Workup
- OGTT if not already done.
- Detailed anatomy scan.
- Doppler and MCA-PSV (fetal anaemia screen).
- Karyotype consideration in severe / unexplained polyhydramnios.
Management
- Treat underlying cause (e.g. glycemic control in GDM).
- Amnioreduction for severe symptomatic polyhydramnios.
- Increased risk of preterm labour — monitor cervical length and symptoms.
Limitations
- Amniotic fluid measurement has 5-10 % inter-observer variability.
- Single time-point values less informative than serial trends.
- Maternal hydration status affects AFI measurably.
- This educational tool implements general thresholds; individual decisions involve full clinical context.
Sources
- Phelan JP, et al. Amniotic fluid volume assessment with the four-quadrant technique at 36-42 weeks’ gestation. J Reprod Med 1987;32:540-2.
- Chamberlain PF, et al. Ultrasound evaluation of amniotic fluid volume. Am J Obstet Gynecol 1984;150:245-9.
- Nabhan AF, Abdelmoula YA. Amniotic fluid index versus single deepest vertical pocket as a screening test for preventing adverse pregnancy outcome. Cochrane Database Syst Rev 2008;3:CD006593.
- ACOG. Committee Opinion 828: Indications for Outpatient Antenatal Fetal Surveillance.
- SMFM. Consult Series on Antenatal Fetal Surveillance.
- Hofmeyr GJ, et al. Amnioinfusion for third trimester preterm premature rupture of membranes. Cochrane (related context).
Frequently asked questions
What's the difference between AFI and SDP?
Both measure amniotic fluid volume on ultrasound. AFI (Amniotic Fluid Index, Phelan 1987) sums the deepest vertical pocket in each of 4 uterine quadrants — typical range 8-18 cm at term, abnormal at ≤ 5 (oligo) or > 25 (poly). SDP (Single Deepest Pocket, Chamberlain 1984) measures just the largest single pocket — normal 2-8 cm. The Nabhan 2008 Cochrane review of 1,635 women found AFI ≤ 5 led to MORE inductions and caesareans without improving outcomes, vs SDP ≤ 2 cm. SMFM and ACOG now recommend SDP for routine surveillance.
What causes oligohydramnios (low fluid)?
Common causes: ruptured membranes (loss of fluid externally), fetal growth restriction with placental insufficiency (reduced fetal urine output), post-term pregnancy (placental aging), fetal renal anomaly (agenesis, dysplasia, obstruction), maternal dehydration (transient effect), maternal medications (NSAIDs, ACE inhibitors). Workup: check rupture of membranes, fetal growth scan, maternal hydration history, medication review, anatomy scan if not done.
What causes polyhydramnios (high fluid)?
Common causes: maternal diabetes (poorly controlled), fetal swallowing impairment (CNS anomaly, oesophageal atresia, tracheoesophageal fistula, neuromuscular disorders), fetal hydrops (anemia, infection, cardiac), monochorionic twin TTTS, idiopathic (~50 %). Mild polyhydramnios is often idiopathic and benign. Severe polyhydramnios increases preterm labour risk and warrants detailed anatomy review and glucose tolerance testing.
Does oligohydramnios mean induction?
Not always. Isolated oligohydramnios at term (37+ weeks) with otherwise reassuring fetal status is a common induction indication. Earlier oligohydramnios at < 36-37 weeks needs more nuanced assessment — is there growth restriction? Is the fetus otherwise well? PROM ruled out? Many centres use SDP rather than AFI to reduce false-positive 'oligohydramnios' and unnecessary induction. Discuss with your obstetric team.
How is amniotic fluid measured?
Transabdominal ultrasound. AFI: probe perpendicular to mother's spine, uterus divided into 4 quadrants by the linea nigra and umbilicus; measure the deepest vertical pocket in each, sum the four values. SDP: identify the largest single pocket free of cord or fetal parts, measure vertical depth. Both should be measured with the woman supine and the bladder empty for consistency.
What's the normal range through pregnancy?
Amniotic fluid increases from ~30 mL at 10 weeks to ~800 mL at 32 weeks, then gradually decreases to ~500 mL at 41 weeks. AFI peaks around 32-34 weeks at a median of 14-16 cm, then falls to ~12 cm at 40 weeks and ~9 cm by 42 weeks. SDP is more stable through gestation, hovering 3-7 cm with a normal threshold of 2-8 cm throughout.
Should I drink more water to increase amniotic fluid?
Possibly. Several small RCTs (Hofmeyr 2002 Cochrane) found that maternal IV hydration with 2 L of hypotonic fluid temporarily raised AFI by ~2 cm in women with oligohydramnios. Oral hydration shows similar but smaller effects. The clinical benefit (does it actually improve outcomes?) is unclear — the change in AFI may reflect maternal volume status rather than true fetal benefit. Worth trying but doesn't replace surveillance and management of underlying causes.