Fetal Growth, Amniotic Fluid & Antepartum Testing
Measure, image, and monitor: deciding which fetus needs delivery, and when.
Size-date discrepancy and fetal growth restriction
Fundal height. From about 20 to 36 weeks, the distance from pubic symphysis to uterine fundus in centimeters approximates gestational age in weeks, with the fundus near the umbilicus at 20 weeks. A discrepancy of more than 3 cm calls for ultrasound to measure fetal biometry and amniotic fluid; obesity and fibroids make the tape unreliable, so those patients go straight to ultrasound. A uterus large for dates suggests wrong dates, macrosomia, polyhydramnios, multiple gestation, or fibroids; small for dates suggests wrong dates, growth restriction, oligohydramnios, fetal demise, or a transverse lie.
Definition. Fetal growth restriction (FGR) is an ultrasound estimated fetal weight or abdominal circumference below the 10th percentile for gestational age. An EFW below the 3rd percentile is severe FGR and carries the highest stillbirth risk. Small for gestational age (SGA) describes a newborn whose birth weight is below the 10th percentile; many small fetuses are constitutionally small and healthy, and normal Doppler studies and fluid help identify them.
Symmetric (early) FGR. Onset before 32 weeks with head and body proportionally small points to an intrinsic fetal problem: aneuploidy (trisomy 18, trisomy 13, triploidy), congenital infection (CMV most often; also toxoplasmosis, rubella, varicella), or teratogens such as alcohol. Evaluation includes a detailed anatomy scan and an offer of diagnostic genetic testing by amniocentesis with chromosomal microarray; amniotic fluid is also sent for CMV PCR.
Asymmetric (late) FGR. Uteroplacental insufficiency limits nutrient supply later in pregnancy. Causes include chronic or gestational hypertension and preeclampsia, pregestational diabetes with vasculopathy, smoking, antiphospholipid syndrome, SLE, renal disease, and multiple gestation. The fetus redistributes blood to the brain (head sparing), so head circumference is preserved while the abdominal circumference lags because the liver loses glycogen, raising the head-to-abdomen ratio.
Doppler surveillance and timing delivery in FGR
Umbilical artery Doppler. A healthy placenta is a low-resistance bed, so the umbilical artery carries forward flow throughout diastole. As placental vessels are lost, resistance rises: diastolic flow falls (higher systolic/diastolic ratio), then becomes absent, and finally reverses. Each step raises the risk of fetal acidemia and stillbirth. Doppler is used in suspected FGR, not in normally grown fetuses, and is combined with NSTs or biophysical profiles and serial growth scans.
| Finding | Deliver at |
|---|---|
| EFW 3rd–10th percentile, normal Doppler | 38–39 weeks |
| EFW <3rd percentile, or decreased diastolic flow | 37 weeks |
| Absent end-diastolic flow | 33–34 weeks |
| Reversed end-diastolic flow | 30–32 weeks |
After birth. Growth-restricted newborns are prone to hypoglycemia (small glycogen and fat stores), hypothermia, polycythemia (chronic hypoxia drives erythropoietin), and meconium aspiration. In adulthood they have higher rates of hypertension, type 2 diabetes, and cardiovascular disease, the fetal origins (Barker) hypothesis.
Macrosomia
Definition and cause. Macrosomia is a birth weight of at least 4,000–4,500 g; large for gestational age means above the 90th percentile. Maternal glucose crosses the placenta but insulin does not, so the fetus secretes its own insulin, which acts as a growth hormone and deposits fat over the shoulders and trunk. Other risk factors are obesity, excess gestational weight gain, post-term pregnancy, a prior macrosomic infant, and male sex.
Complications. Shoulder dystocia with brachial plexus injury (Erb palsy) or clavicle fracture, cesarean delivery, postpartum hemorrhage, severe perineal lacerations, and neonatal hypoglycemia in infants of diabetic mothers.
Management. Late-pregnancy ultrasound estimates can be off by 15–20%, so suspected macrosomia alone does not justify induction before 39 weeks. A planned cesarean is offered when the EFW is at least 5,000 g in a woman without diabetes, or at least 4,500 g in a woman with diabetes.
Amniotic fluid disorders
Physiology. In the second half of pregnancy, amniotic fluid is mostly fetal urine, with a contribution from lung fluid, and it is removed mostly by fetal swallowing. Low fluid therefore means reduced urine output or a leak; high fluid means impaired swallowing or absorption, or excess urine.
| Feature | Oligohydramnios | Polyhydramnios |
|---|---|---|
| Definition | AFI ≤5 cm or deepest vertical pocket <2 cm | AFI ≥24 cm or deepest vertical pocket ≥8 cm |
| Causes | Rupture of membranes (most common); placental insufficiency (FGR, preeclampsia); post-term; bilateral renal agenesis; urinary obstruction (posterior urethral valves); polycystic kidneys; NSAIDs, ACE inhibitors, ARBs | Idiopathic (most common); maternal diabetes (fetal hyperglycemia → polyuria); GI atresia (esophageal, duodenal); impaired swallowing (anencephaly, neuromuscular disease); TTTS recipient; fetal anemia or hydrops |
| Consequences | Pulmonary hypoplasia, limb contractures, clubfoot, Potter facies; cord compression with variable decelerations | Preterm labor, PPROM, malpresentation, cord prolapse, abruption at rupture, uterine atony with PPH, maternal dyspnea |
| Management | Treat the cause; isolated, persistent oligohydramnios → deliver at 36 0/7–37 6/7 weeks | Anatomy scan, diabetes testing, antibody and infection studies if hydrops; amnioreduction for severe symptoms |
Oligohydramnios. The deepest vertical pocket is preferred over the AFI for diagnosis, because it leads to fewer unnecessary inductions without worse outcomes. Lung development depends on fluid in the second trimester, which is why early, prolonged oligohydramnios causes pulmonary hypoplasia. ACE inhibitors and ARBs damage the fetal kidneys in the second and third trimesters (oligohydramnios, renal failure, skull hypoplasia), so a pregnant patient taking one is switched to labetalol or nifedipine.
Polyhydramnios. A double bubble on ultrasound points to duodenal atresia, and an absent stomach bubble to esophageal atresia. Mild idiopathic polyhydramnios is not an indication for delivery before 39 weeks.
Indomethacin is not recommended solely to reduce amniotic fluid volume, because it constricts the fetal ductus arteriosus and reduces fetal renal output.
Antepartum fetal testing
Who is tested. Surveillance is offered when stillbirth risk is increased: pregestational diabetes or gestational diabetes treated with medication, hypertensive disorders, FGR, oligohydramnios or polyhydramnios, decreased fetal movement, post-term pregnancy (41 weeks or more), prior stillbirth, monochorionic twins, SLE or antiphospholipid syndrome, and CKD. Testing usually begins around 32 weeks, earlier when risk is high, and is repeated weekly or twice weekly. Decreased fetal movement after viability warrants a prompt NST.
| Test | Reassuring | Abnormal and next step |
|---|---|---|
| Nonstress test | Reactive: ≥2 accelerations ≥15 bpm for ≥15 s in 20 min (≥10 bpm for ≥10 s before 32 weeks) | Nonreactive after 40 min and vibroacoustic stimulation → BPP or CST |
| Biophysical profile | 8–10 with normal fluid | 6 equivocal: repeat within 24 h, or deliver at term; ≤4 → usually deliver |
| Modified BPP | Reactive NST and AFI >5 cm | Either abnormal → full BPP or CST |
| Contraction stress test | Negative: no late or significant variable decelerations | Positive: late decelerations after ≥50% of contractions → deliver |
Nonstress test. Accelerations reflect an intact, well-oxygenated autonomic nervous system, so a reactive NST reliably excludes current acidemia. Fetal sleep cycles can last up to 40 minutes, so a quiet tracing is extended; vibroacoustic stimulation wakes the fetus and shortens testing without losing accuracy.
Biophysical profile. Over 30 minutes, five items each score 2 or 0:
- NST: reactive.
- Breathing: at least one episode of rhythmic breathing lasting ≥30 seconds.
- Movement: at least three discrete body or limb movements.
- Tone: at least one extension with return to flexion.
- Fluid: a deepest vertical pocket of 2 cm or more.
The NST, breathing, movement, and tone are acute markers of oxygenation, while fluid reflects chronic placental function; oligohydramnios therefore needs further evaluation even when the total score is 8/10. The modified BPP pairs the NST (acute) with the AFI (chronic).
Contraction stress test. Oxytocin or nipple stimulation produces at least three contractions in 10 minutes to see whether the placenta can oxygenate the fetus during stress. It is avoided whenever labor would be unsafe (placenta previa, vasa previa, prior classical cesarean) and with PPROM or high risk of preterm labor.