Why the same mistakes keep happening
Most pregnancy dating errors are not arithmetic errors — they are assumption errors. The formula is right, but the input is idealized: a round number, an optimistic rate, a best-case month. Below are the mistakes behind most bad results, grouped by the calculator where they bite.
Due Date Calculator
Treating the due date as a deadline — the 37–42 week window is the actual plan.
Using cycle-adjusted dates after ultrasound re-dating; keep one official date from your provider.
Recalling LMP loosely (spotting vs period) — when uncertain, ultrasound dating wins.
Pregnancy Week Calculator
Confusing gestational weeks with weeks since conception — always two weeks apart.
Tracking from the due date AFTER a provider re-dated it — use the official date only.
Counting the due date as an endpoint; week 40 is the center of the term window, not the deadline.
IVF Due Date Calculator
Dating from egg retrieval or trigger dates instead of the transfer — the embryo age at transfer is the anchor.
Adding 280 days from transfer (the LMP rule) — that overcounts by the embryo's age at transfer.
Second-guessing the clinic's date with cycle-based calculators — IVF dating is strictly more precise.
Conception Date Estimator
Assuming conception = the day of intercourse — sperm persistence makes earlier days the likely winners.
Using cycle-day math for paternity questions — those need professional genetic testing, not arithmetic.
Applying day-14 ovulation to irregular cycles without adjusting — the assumption is the error.
The habit that fixes all of them
Write down the assumption you are least sure about every time you run a number. If the answer matters, test it: change that one input by ±20% and see whether the decision flips. If it flips, the assumption — not the math — is your real problem, and it deserves the research time.