Pregnancy Dating
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Direct answer
Pregnancy dating establishes the estimated delivery date, and the crown-rump length between 8 weeks 0 days and 13 weeks 6 days is the most accurate tool (within 5-7 days), superseding the last menstrual period whenever the two disagree by more than the margin — once assigned, the date is never changed. The last menstrual period works when the cycle is regular (about 28 days), the dates are certain, and there has been no contraception or bleeding since; Naegele's rule adds nine months and seven days to the first day, corrected for cycle length (add or subtract the difference from 28). Assisted conceptions date themselves: in-vitro fertilisation dates from the transfer day plus two weeks for oocyte age.
What you must remember
- Naegele's rule: first day of the last period plus 9 months plus 7 days; a 32-day cycle moves the date 4 days later, a 26-day cycle 2 days earlier.
- Gestational age convention: counts from the first day of the last menstrual period, roughly two weeks before conception — a distinction the exam probes with "embryonic age versus gestational age".
- Crown-rump length window: 7-13+6 weeks (many protocols 8-13+6), accuracy 5-7 days; by 84 mm (about 14 weeks) biometrics take over.
- Redating thresholds (ACOG-style rules): before 9 weeks, redates if the scan differs by more than 5 days; 9 to 13+6 weeks, more than 7 days; second trimester, more than 10-14 days; third trimester, more than 21 days — after first-trimester assignment, the date is fixed.
- Second-trimester biometrics: biparietal diameter and head circumference (best at 14-26 weeks), femur length, abdominal circumference — combined error 10-14 days, widening to 21 or more in the third trimester because size, not age, is measured.
- Certain-dates checklist: period recalled to the day, regular cycles, no hormonal contraception or lactational amenorrhoea before, no first-trimester bleeding — a single miss demotes the dates.
- Self-dating pregnancies: in-vitro fertilisation (transfer date plus 2 weeks for gestational age), intrauterine insemination and ovulation trigger dates — more accurate than any measurement.
- Why dating discipline matters: induction thresholds (41 weeks), preterm definitions, corticosteroid and magnesium windows, and growth screening all key off the assigned date — a wrong date cascades downstream.
Dating three different women
First, a 25-year-old with certain dates, regular 28-day cycles, last menstrual period 10 January: Naegele gives 17 October; a first-trimester scan on 1 March shows a crown-rump length matching 7+6 weeks, agreeing within days — the dates stand.
Second, a 29-year-old with polycystic ovary syndrome, cycles of 35-60 days, last period "around six months ago". Her dates fail every criterion; the first-trimester crown-rump length is the sole reference — she is dated 7+4 weeks when she believed 9-10, changing her screening schedule. Third, a 33-year-old conceiving by in-vitro fertilisation with a day-5 transfer on 5 April: gestational age that day is 2+5 weeks, and the estimated delivery date follows arithmetically — no later scan can override the embryology.
The fourth situation, common in India: booking at 24 weeks with no first-trimester scan. Dating rests on second-trimester biometry with an error up to two weeks — why programme guidance schedules a first-trimester scan, and why "dating scan" and "anomaly scan" are never interchangeable terms.
How the exam frames it
Three item types recur: arithmetic — Naegele's rule with a cycle correction, or a crown-rump length table; hierarchy — most accurate is the first-trimester crown-rump length, then certain last menstrual period, then second-trimester biometry; and rules — when a date is reassigned (over 5 days discrepant before 9 weeks, and so on) and when never (after first-trimester assignment). The conceptual favourite is gestational versus embryonic age (two weeks apart), with the in-vitro fertilisation transfer-date twist applied. Expect also which measurement replaces crown-rump length after 14 weeks (biparietal diameter or composite biometry).
Frequently asked questions
What is the most accurate method of dating a pregnancy?
The crown-rump length on ultrasound between about 8 and 13 weeks 6 days, accurate to within 5-7 days — more reliable than any recollection of the last menstrual period.
How is Naegele's rule applied and corrected?
Add 9 months and 7 days to the first day of the last menstrual period, then adjust for cycle length — one day added or removed for each day the cycle differs from 28.
When should an assigned due date be changed?
Only before the dating is established: a discrepancy over 5 days before 9 weeks or over 7 days at 9-13+6 weeks redates the pregnancy; after a first-trimester assignment, the date stands.
How is pregnancy dated after in-vitro fertilisation?
From the embryo transfer — a day-5 blastocyst transfer corresponds to a gestational age of 2 weeks 5 days that day, giving a due date of transfer day plus 261 days.
Why does gestational age not equal embryonic age?
Gestational age is counted from the first day of the last menstrual period, roughly two weeks before ovulation and conception, so the embryo is always about two weeks "younger" than the pregnancy.