A 26-year-old woman, gravida 3, para 2, presents for her 1st antenatal visit.

Comprehensive and Detailed Explanation:

At 20 weeks’ gestation, the uterine fundus typically reaches the umbilicus. If the fundal height is lower than expected at a stated gestational age of 26 weeks, the most likely explanation—especially with no prior care—is a dating error.

Toronto Notes 2023 – Obstetrics:

“Fundal height at the umbilicus corresponds to ~20 weeks. In women lacking early prenatal care, dating errors are common causes of fundal height discrepancy.”

MCCQE1 Objectives (Obstetrics > 80-3: Routine Antenatal Care):

“Candidates must interpret fundal height and reconcile discrepancies with gestational age.”

IUGR and oligohydramnios are possible but require imaging confirmation. Bicornuate uterus may cause fundal shape anomalies but not significant dating discrepancy. A mole (D) would increase, not decrease, size.

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