Accurate dating anchors every downstream decision: growth percentiles, timing of screening, delivery thresholds, BPP interpretation, and preterm-labour cutoffs all depend on GA. A redating error of even a few days at term can misclassify post-dates management. The dating hierarchy exists because LMP recall is unreliable in many patients (irregular cycles, missed bleeds, recent hormonal contraception), while earliest ultrasound is most accurate. Robinson-Fleming CRL at 7–12 wk is accurate to within ±5–7 days; by T3, biometry dating error spreads to ±3 weeks.

AC is the most sensitive single parameter for fetal growth restriction — before EFW crosses the 10th percentile, AC often does. An isolated AC < 10th %ile with normal Doppler still warrants follow-up; it is a recognised flag for early placental dysfunction (ISUOG 2020).
Symmetric vs asymmetric growth restriction matters: asymmetric (elevated HC/AC, brain-sparing) suggests placental cause and is usually late-onset; symmetric suggests earlier insult (infection, aneuploidy, constitutional).
Choice of standard matters too — a Hadlock-10th-percentile fetus may be at the INTERGROWTH-3rd percentile. Canadian SOGC #442 (2023) still permits local or Hadlock references; ISUOG 2020 recommends a prescriptive standard where available.

SDP is preferred over AFI — the SMFM Consult Series #67 (2022), Cochrane 2008, and SOGC all agree: AFI over-diagnoses oligohydramnios and polyhydramnios without reducing perinatal morbidity. Interventions (inductions, sections) triggered by AFI alone expose mother and fetus to iatrogenic harm.
Oligohydramnios reflects the fetal urine-output pathway: think ROM, renal agenesis/dysplasia, lower-tract obstruction, severe placental insufficiency (FGR). A comprehensive anatomy survey (especially kidneys and bladder) is essential.
Polyhydramnios reflects impaired swallowing, increased urine output, or membrane pathology: GI obstruction (TEF, duodenal atresia), neuromuscular disorders, fetal hydrops, maternal GDM, multiples (TTTS), infection (CMV, syphilis, parvovirus). Idiopathic in ~60% of mild cases — but mild poly that progresses warrants work-up.

UA Doppler drives delivery timing. In early-onset FGR (<32 wk), the TRUFFLE trial showed DV PI-based timing improves survival without disability vs CTG-based timing. In late-onset FGR, CPR (MCA PI / UA PI) <5th %ile is the key marker: it identifies brain-sparing before UA becomes abnormal and is linked to stillbirth risk and adverse neonatal outcome even in AGA fetuses.
UCR (UA PI / MCA PI, Flatley & DeVore 2019) is being proposed as a more sensitive alternative to CPR in late pregnancy because its distribution is more symmetric. We show both — CPR remains the guideline-referenced index.
MCA PSV ≥ 1.5 MoM (Mari 2000) predicts moderate-severe fetal anemia with ~100% sensitivity and drives IUT discussion.
UtA PI in T1 feeds FMF's multi-marker pre-eclampsia screen (combined with MAP, PlGF, PAPP-A); high in T2 predicts late-onset PE and FGR.

The combined test's strength is that no single marker makes or breaks it — each has a likelihood ratio that shifts the patient's background (age-derived) risk up or down. The FMF algorithm combines them multiplicatively.
NT > 3.5 mm (≥99th %ile) is independently important even if aneuploidy is ruled out — it flags 10–30% of structural anomalies, especially cardiac, and warrants detailed anatomy + fetal echo at 20–22 wk.
cfDNA (NIPT) has largely superseded combined-test screening for high-risk indications — detection for T21 ≥99%, T18 ~97%, T13 ~87%. Combined test retains value in low-resource settings and flags the non-trisomy NT-linked pathway.

25 mm at 16–24 wk is the cutoff for intervention in both singletons and twins (EPPPIC IPD meta-analysis 2021). 15 mm marks very-short cervix with ~ 50% PTB < 32 wk risk.
Cerclage is evidence-based only in singletons with prior sPTB + current short CL — ultrasound-indicated cerclage (NNT ~8 per MRC/Berghella). Routine cerclage in twins or in singletons without prior PTB is not supported.
Funneling adds no independent prognostic value beyond the remaining CL; measure the still-closed portion only.

As fetal hypoxia develops, components disappear in the reverse order of ontogeny: breathing lost first (~CNS centre at 20–21 wk), then movement, tone, and finally heart-rate reactivity (~26–28 wk centre). AFV reflects chronic hypoxia (redistribution → reduced renal perfusion → oligohydramnios). So oligo + otherwise reassuring acute components = chronic insult; reassuring fluid + multiple acute findings lost = acute insult. Either trumps an unweighted score.

DCDA has essentially-independent placentas and behaves like two singletons; MCDA shares vascular anastomoses and therefore risks TTTS (fluid discordance), sFGR (unequal placental share with asymmetric flow), and TAPS (gradual red-cell transfer without fluid shift). Get chorionicity right in the first trimester (λ vs T sign, membrane count) — mis-assignment makes the rest of surveillance meaningless.