Dating a Pregnancy Accurately: Where the Errors Hide

Up to about 14 weeks, ultrasound dating is precise to within a few days and is accepted as the clinical standard for correcting uncertain dates. Between 14 and 24 weeks it remains the best available measure but with growing uncertainty. After 24 weeks, dating relies on earlier records, which is why settling the question early matters so much.

Before 14 weeks, the scan wins in essentially every guideline — first-trimester ultrasound is more reliable than any period-based estimate. Later in pregnancy the balance shifts: if your early dating was firm (IVF, tracked ovulation), a later scan that disagrees usually reflects the baby's individual growth rather than a wrong due date.

Yes — with a short cycle (say 23 days), ovulation happens around day 9, so pregnancy begins almost a week earlier than a 28-day calculator claims. The corrected estimate in our due date calculator handles this automatically when you enter your real cycle length.

Usually not — but the exceptions are sharp. Screening windows (nuchal translucency at 11w0d–13w6d, anatomy scan at 18–22 weeks, glucose screening at 24–28 weeks) have real edges, and interpreting a growth scan requires knowing which week you are actually in. A few days is noise; a week or more is a conversation worth having with your provider.