Due date accuracy: ultrasound vs. last period

How do the options compare?

The lens is a clinician's: guideline endorsement, pregnancy evidence, dosing, onset, and tolerability. Cost and formulation notes appear where relevant.

• A first-trimester ultrasound using crown-rump length, done between 8 and 13 weeks 6 days, is the single most accurate way to date a pregnancy, typically within about 5 to 7 days of the true gestational age. 1
• Clinical guidance sets specific redating thresholds: before 9 weeks, a last-period-based date is only kept if it falls within 5 days of the ultrasound estimate; from 9 to 13 weeks 6 days the allowed gap widens to 7 days, and beyond that the ultrasound date takes over. 1
• Ultrasound dating becomes progressively less precise later in pregnancy: the margin of error widens to roughly 7 to 10 days in the early second trimester, 10 to 14 days by the late second trimester, and 21 to 30 days by the third trimester, which is why late scans aren't used alone to reset a due date. 2
• The World Health Organization's antenatal care guidance recommends an ultrasound before 24 weeks partly to establish gestational age, calling first-trimester ultrasound the most accurate method available and noting how much accurate dating matters for timing later interventions. 3
• An Indian cohort screened 14,701 pregnancies, but only 10,018 participants could recall their last menstrual period. Among that subgroup, last-period and ultrasound dates agreed in about 70% under the study criteria, illustrating the limits of relying on menstrual dates alone. 4

Which option makes sense?

The comparison above is intentionally evidence-first, not brand-first. Please consult your healthcare provider to translate it into a plan that fits your pregnancy specifics.

Disclosure: Pregnancy Smart is a supplement maker. The options compared above sit outside our own product line; this page describes their pregnancy evidence on its own merits.