1.4 Antepartum Fetal Surveillance, Labor Progression & Perinatal Screening
Key Takeaways
- A nonstress test is reactive at ≥ 32 weeks with 2 accelerations of ≥ 15 bpm lasting ≥ 15 seconds in 20 minutes, but below 32 weeks the criterion drops to 10 bpm for 10 seconds — applying the term criterion to a preterm fetus produces false nonreactive results.
- Each biophysical profile component scores 2 or 0 (never 1) and amniotic fluid volume is the chronic marker: oligohydramnios (AFI ≤ 5 cm or deepest pocket < 2 cm) predicts pulmonary hypoplasia, contractures, and cord compression, while polyhydramnios (AFI ≥ 24 cm or pocket ≥ 8 cm) predicts a fetus who cannot swallow — esophageal or duodenal atresia, anencephaly, or maternal diabetes.
- ACOG's acid-base criterion for a possible acute intrapartum hypoxic-ischemic event requires BOTH an umbilical arterial pH < 7.0 AND a base deficit ≥ 12 mmol/L; an isolated low pH with a normal base deficit is usually a transient respiratory acidosis with an excellent prognosis.
- Universal GBS vaginal-rectal culture is collected at 36 0/7 to 37 6/7 weeks, and prophylaxis counts as adequate only when penicillin, ampicillin, or cefazolin was given at least 4 hours before delivery — clindamycin and vancomycin never count as adequate.
- Vacuum extraction risks a subgaleal hemorrhage that crosses suture lines, shifts with position, and can sequester 20 to 40% of blood volume, whereas forceps classically produce facial nerve palsy; serial head circumference measurement is the key nursing surveillance after any vacuum delivery.
1.4 Antepartum Fetal Surveillance, Labor Progression & Perinatal Screening
Clinical Pearl & Core Takeaway: The NCC blueprint lists Fetal Assessment, Nonstress testing, Biophysical profile, Antepartal screening, Cord gases and Complications of Labor as named General Assessment topics. You will not be asked to perform these tests — you will be asked what a given result means for the baby you are about to receive. Read every antepartum result as a prediction: a nonreactive NST plus oligohydramnios predicts an infant who tolerates labor poorly; a cord arterial pH of 6.95 with a base deficit of 15 predicts an infant who needs an encephalopathy exam within the hour.
1. The Antepartal Screening Timeline
Before you can interpret a maternal chart you need to know what is normally in it and when it was obtained. The prenatal record is the single richest source of neonatal risk data available to you at delivery.
| Gestational Window | Screening / Test | What an Abnormal Result Predicts for the Neonate |
|---|---|---|
| First visit | Blood type, Rh, antibody screen, CBC, rubella immunity, hepatitis B surface antigen, HIV, syphilis (RPR/VDRL), gonorrhea and chlamydia NAAT, urine culture | Rh alloimmunization → hemolytic disease; HBsAg+ → hepatitis B immune globulin plus vaccine within 12 hours; untreated syphilis → congenital syphilis workup; GBS bacteriuria → automatic intrapartum prophylaxis |
| 10–13 weeks | Cell-free DNA (cfDNA) screening; nuchal translucency ultrasound | High-risk cfDNA for trisomy 21, 18, or 13 → anticipate dysmorphology, cardiac defects, feeding difficulty |
| 15–22 weeks | Quadruple screen (AFP, hCG, unconjugated estriol, inhibin A) | Elevated maternal serum AFP → neural tube defect or abdominal wall defect (omphalocele/gastroschisis); low AFP → aneuploidy |
| 18–22 weeks | Detailed fetal anatomy ultrasound | Structural anomalies drive the entire delivery-room plan — a prenatally diagnosed diaphragmatic hernia, gastroschisis, or ductal-dependent lesion should never be a delivery-room surprise |
| 24–28 weeks | 1-hour 50 g glucose challenge → 3-hour 100 g tolerance test if abnormal | Gestational diabetes → macrosomia, birth trauma, hypoglycemia, polycythemia, hypocalcemia |
| 28 weeks | Repeat antibody screen; anti-D immune globulin for Rh-negative mothers | Failure to give anti-D → sensitization risk in a subsequent pregnancy |
| 36 0/7 – 37 6/7 weeks | Vaginal–rectal GBS culture | Positive culture → intrapartum antibiotic prophylaxis; inadequate prophylaxis → early-onset sepsis surveillance |
Exam trap: the GBS screening window is 36 0/7 to 37 6/7 weeks. The older 35–37 week window is still printed in many hospital binders and third-party review books; ACOG moved it in 2019 (Committee Opinion 782, superseded by 797) so that the culture stays valid through 41 weeks, because a GBS culture is only considered reliable for five weeks.
2. The Nonstress Test (NST)
The NST asks one question: is the fetal autonomic nervous system intact and well-oxygenated enough to accelerate the heart rate when the fetus moves? An intact acceleration requires a functioning cerebral cortex, medulla, and cardiac conduction system — which is why a reactive test is such a strong predictor of adequate oxygenation.
Interpretation criteria (memorize the two-tier rule)
| Gestational Age | Reactive requires | Rationale |
|---|---|---|
| ≥ 32 weeks | 2 or more accelerations of ≥ 15 bpm above baseline, each lasting ≥ 15 seconds, within a 20-minute window ("15 by 15") | Mature autonomic control |
| < 32 weeks | 2 or more accelerations of ≥ 10 bpm lasting ≥ 10 seconds in 20 minutes ("10 by 10") | The immature fetus physiologically cannot mount a 15 × 15 acceleration; scoring it against the term criterion produces false-positive nonreactive results |
- A nonreactive NST is not by itself a diagnosis of hypoxia. The commonest cause is a fetal sleep cycle, which can last 20–40 minutes. The test is therefore extended to 40 minutes, and vibroacoustic stimulation may be applied to rouse the fetus before the test is called nonreactive.
- Other benign causes: maternal sedation or opioid analgesia, magnesium sulfate, nicotine, and prematurity.
- A genuinely nonreactive NST escalates to a biophysical profile or contraction stress test.
3. The Biophysical Profile (BPP)
The BPP combines the NST with four real-time ultrasound observations. Each component scores 2 (present) or 0 (absent) — there is no score of 1.
| Component | Scores 2 when… | Physiologic centre being tested |
|---|---|---|
| Nonstress test | Reactive by the criteria above | Posterior hypothalamus / medulla (last to develop, first to fail with hypoxia) |
| Fetal breathing movements | ≥ 1 episode of rhythmic breathing lasting ≥ 30 seconds within 30 minutes | Ventral surface of the 4th ventricle |
| Gross body movements | ≥ 3 discrete body or limb movements in 30 minutes | Cortex / nuclei |
| Fetal tone | ≥ 1 episode of extension of an extremity or spine with return to flexion, or opening/closing of a hand | Cortical subcortical area (earliest to develop, last to fail) |
| Amniotic fluid volume | A single deepest vertical pocket > 2 cm | Chronic marker — reflects fetal renal perfusion over days to weeks |
Score interpretation
- 8 to 10 — normal; risk of fetal asphyxia within one week is very low.
- 6 — equivocal; repeat in 24 hours or deliver depending on gestational age.
- 4 — abnormal; strongly suggests fetal compromise, usually prompting delivery.
- 0 to 2 — immediate delivery.
The single most important nursing insight: the four acute markers (NST, breathing, movement, tone) reflect fetal status right now, while amniotic fluid volume is the chronic marker. A score of 8/10 that lost its two points for low fluid is far more ominous than a score of 8/10 that lost its two points for a nonreactive NST, because oligohydramnios means the fetus has been shunting blood away from the kidneys for days or weeks. The modified BPP (NST + amniotic fluid index only) is used as a rapid screen for exactly this reason.
4. Amniotic Fluid Disorders
Amniotic fluid after ~16 weeks is essentially fetal urine, and it is swallowed and recirculated. Volume abnormalities are therefore a direct readout of fetal renal output and gastrointestinal patency.
| Condition | Quantitative Definition | Common Fetal / Neonatal Causes | What to Anticipate at Delivery |
|---|---|---|---|
| Oligohydramnios | AFI ≤ 5 cm or single deepest pocket < 2 cm | Renal agenesis (Potter sequence), posterior urethral valves, bilateral multicystic dysplastic kidneys, prolonged rupture of membranes, uteroplacental insufficiency, post-term pregnancy, indomethacin exposure | Pulmonary hypoplasia (fluid is required for lung growth), limb contractures and positional deformities, flattened facies, cord compression with variable decelerations, meconium-stained fluid; anticipate a difficult resuscitation with poor lung compliance |
| Polyhydramnios | AFI ≥ 24 cm or single deepest pocket ≥ 8 cm | Anything that stops the fetus swallowing: esophageal atresia / tracheoesophageal fistula, duodenal atresia, anencephaly and other neural tube defects, severe neuromuscular disease; also poorly controlled maternal diabetes (osmotic fetal diuresis), fetal anemia/hydrops, twin-to-twin transfusion | Pass an orogastric tube in the delivery room if esophageal atresia is suspected (coiling at ~10 cm is diagnostic); watch for the "double bubble" of duodenal atresia; expect preterm labor, malpresentation, and cord prolapse at membrane rupture |
High-yield association: polyhydramnios + a newborn with excessive frothy oral secretions and choking at the first feed = esophageal atresia until proven otherwise. Keep the infant NPO, place a sump tube to low intermittent suction, and elevate the head of the bed.
5. Umbilical Cord Blood Gases
Cord gases are the only objective, retrospective measure of the fetal acid–base state at the moment of birth, and the NCC blueprint names them explicitly.
Collection technique
Immediately after delivery, doubly clamp a 10–20 cm segment of cord and draw from the isolated segment with heparinized syringes. Values in a clamped segment remain stable for roughly 60 minutes at room temperature. Always draw the artery first — arteries are the two smaller, thick-walled, coiled vessels; the vein is the single large thin-walled vessel.
Which vessel means what
- Umbilical artery carries blood from the fetus → it reflects fetal tissue metabolism. This is the clinically meaningful sample.
- Umbilical vein carries blood to the fetus from the placenta → it reflects maternal/placental oxygen delivery.
- The artery therefore has a lower pH, lower PO2, and higher PCO2 than the vein. If the two samples are nearly identical, suspect that both were drawn from the same vessel and the "arterial" value cannot be trusted.
| Value | Normal Umbilical Artery | Normal Umbilical Vein |
|---|---|---|
| pH | 7.18 – 7.38 (mean ≈ 7.27) | 7.25 – 7.45 (mean ≈ 7.34) |
| PCO2 (mmHg) | 45 – 55 | 35 – 45 |
| PO2 (mmHg) | 15 – 25 | 25 – 35 |
| Base deficit (mmol/L) | < 12 | < 12 |
The threshold that matters
ACOG defines the acid–base criterion for a possible acute intrapartum hypoxic-ischemic event as an umbilical arterial pH < 7.0 and a base deficit ≥ 12 mmol/L. Both halves are required — an isolated low pH with a normal base deficit usually reflects a respiratory acidosis from transient cord compression, which resolves within minutes of effective ventilation and carries an excellent prognosis. A metabolic acidosis (low pH with a large base deficit) means the fetus ran anaerobic metabolism long enough to accumulate lactate, and that is the infant who needs an urgent encephalopathy assessment and therapeutic hypothermia screening.
6. Group B Streptococcus Screening and Intrapartum Prophylaxis
GBS is the leading cause of early-onset neonatal sepsis, and the adequacy of maternal prophylaxis drives the newborn's entire management pathway.
Who gets intrapartum antibiotic prophylaxis (IAP)?
- Positive vaginal–rectal culture at 36 0/7 – 37 6/7 weeks.
- GBS bacteriuria at any concentration at any point in the pregnancy — this signals heavy colonization and requires IAP without any further culture.
- A previous infant with invasive GBS disease — IAP regardless of this pregnancy's culture.
- Unknown GBS status plus any of: gestation < 37 weeks, rupture of membranes ≥ 18 hours, or intrapartum temperature ≥ 38.0°C (100.4°F).
Regimens
- Penicillin G (drug of choice): 5 million units IV loading dose, then 2.5 to 3.0 million units IV every 4 hours until delivery.
- Ampicillin (alternative): 2 g IV load, then 1 g IV every 4 hours.
- Low-risk penicillin allergy: cefazolin 2 g IV load, then 1 g every 8 hours.
- High-risk allergy (anaphylaxis): clindamycin 900 mg IV every 8 hours only if the isolate is documented susceptible; otherwise vancomycin.
The four-hour rule
Adequate IAP means penicillin, ampicillin, or cefazolin was given at least 4 hours before delivery. Anything less — including a full dose given 90 minutes before birth — is inadequate, and clindamycin and vancomycin are never counted as adequate for the purpose of newborn risk assessment. This single number determines whether the infant is observed for 36–48 hours or is worked up, so it is a favourite exam discriminator.
7. Labor Progression and Abnormal Labor Patterns
The NCC outline lists abnormal labor patterns under Complications of Labor. You need the definitions because they explain why the infant in front of you looks the way it does.
Contemporary definitions
- The active phase of labor begins at 6 cm, not 4 cm (ACOG/SMFM Obstetric Care Consensus). Applying the old 4 cm threshold labels normal latent labor as "arrest" and drives unnecessary cesareans.
- Protracted active phase: cervical dilation slower than expected, but still progressing.
- Arrest of dilation: ≥ 6 cm dilated with ruptured membranes and no cervical change for ≥ 4 hours of adequate contractions, or ≥ 6 hours of inadequate contractions despite oxytocin.
- Arrest of descent / prolonged second stage: no descent after ≥ 3 hours of pushing in a nullipara (≥ 4 hours with an epidural) or ≥ 2 hours in a multipara (≥ 3 hours with an epidural), provided fetal status is reassuring.
Why this reaches the newborn
Prolonged labor and prolonged rupture of membranes raise the risk of chorioamnionitis and therefore of early-onset sepsis. Prolonged second stage with a large fetus raises the risk of shoulder dystocia, and with it clavicular fracture and brachial plexus injury. Prolonged compression of the head produces exaggerated molding, caput succedaneum, and occasionally a subgaleal bleed.
Operative vaginal birth
| Instrument | Characteristic Neonatal Injuries |
|---|---|
| Vacuum extractor | Chignon (a benign artificial caput at the cup site), cephalohematoma (subperiosteal — does not cross suture lines), subgaleal hemorrhage (the dangerous one — a boggy, shifting, dependent swelling that does cross suture lines and can hold 20–40% of the infant's blood volume), retinal hemorrhage, and later hyperbilirubinemia from resorbing blood |
| Forceps | Facial nerve (CN VII) palsy — asymmetric crying facies with the affected side failing to close the eye or move the lower lip; facial bruising and abrasions; corneal abrasion; rarely depressed skull fracture |
Nursing priority after any vacuum delivery: serial head circumference measurements and repeated palpation of the scalp. A subgaleal hemorrhage announces itself as a rising head circumference, a scalp that feels like a fluid-filled bag, ears displaced anteriorly, and pallor with tachycardia — it is a haemorrhagic emergency, not a cosmetic finding.
A 30-week fetus undergoes a nonstress test. Over 20 minutes the monitor records two accelerations, each rising 12 bpm above baseline and lasting 12 seconds. The nurse is asked how this test should be classified. What is the correct interpretation?
A term infant is delivered by emergency cesarean for a prolonged deceleration. Umbilical arterial cord gas results are pH 7.04, PCO2 78 mmHg, and base deficit 5 mmol/L. The infant required brief positive pressure ventilation and now has a normal neurologic examination at 20 minutes of life. How should the nurse interpret these values?
A multiparous woman at 39 weeks with a positive GBS culture is admitted in active labor. She receives one 5 million unit dose of intravenous penicillin G and delivers 2 hours later. The infant is term, vigorous, and asymptomatic. How should the nurse characterize the adequacy of intrapartum prophylaxis?