Dating of Pregnancy
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Direct answer
Dating establishes gestational age and the expected date of delivery, and it is counted from the first day of the last menstrual period, not from conception. With a certain last period, a regular cycle and no recent hormonal contraception, Naegele's rule gives the due date as the first day of the last period plus nine months and seven days. First-trimester ultrasound, measuring crown-rump length, is the most accurate method (within about five to seven days) and re-dates the pregnancy whenever the menstrual estimate disagrees beyond set limits, because every later decision hangs on correct dates.
What you must remember
- Naegele's rule: add nine months and seven days to the first day of the last menstrual period (equivalently subtract three months and add seven days, then add one year); it presumes a 28-day cycle with ovulation on day 14.
- Irregular cycles, recent oral contraceptive use, lactation or a first-trimester bleed make the last period unreliable; adjust by cycle length or default to ultrasound.
- Fundal height landmarks: at 12 weeks the uterus is just palpable above the symphysis pubis, at 20 weeks it reaches the umbilicus, and at 36 weeks it touches the costal margin (xiphisternum) before lightening drops it slightly; after 20 weeks the symphysiofundal height in centimetres roughly equals gestational weeks.
- Ultrasound accuracy: crown-rump length in the first trimester is accurate to about 5-7 days; second-trimester biometry to about 7-10 days up to 20 weeks, 14 days up to 28 weeks and 21-30 days in the third trimester.
- Re-dating rule: keep the menstrual date when the ultrasound estimate falls within these margins; re-date when the discrepancy exceeds the trimester-specific limit; the earliest available first-trimester scan is the reference standard.
- Terminology: gestational age counts from the last period and exceeds fertilisation age by about two weeks; term is 37-42 weeks (early term 37-38, full term 39-40, late term 41, post-term 42 completed weeks and beyond).
- Clinical uses: dating fixes windows for the nuchal scan, the anomaly survey, gestational diabetes screening, corticosteroids and the definition of post-term pregnancy.
Common confusion
The commonest slip is confusing gestational age with conception (fertilisation) age — the former is two weeks ahead because it starts at the last period. The second is over-relying on third-trimester ultrasound: late biometry measures size, not age, so a late size-date discrepancy should trigger growth assessment, not re-dating. Naegele's rule fails silently in irregular cycles; adjust for follicular phase length or accept the first-trimester scan date.
Exam-focused takeaway
Numerical stems dominate: apply Naegele's rule to a given date, match fundal height to weeks, and pick the most accurate dating method per trimester. Vignettes present an unsure last period and ask for the next step — a first-trimester scan or, in the second trimester, biometry-based dating. Expect a definitional one-liner on post-term pregnancy.
Frequently asked questions
What is Naegele's rule?
The expected date of delivery is the first day of the last menstrual period plus nine months and seven days, assuming a 28-day cycle.
Which is the most accurate method of dating?
First-trimester ultrasound measurement of the crown-rump length, accurate to within about five to seven days.
When should an ultrasound date override the last period?
When the discrepancy exceeds the trimester-specific margin — about 7-10 days in the first and early second trimester, about 14 days up to 28 weeks.
At what week does the uterus reach the umbilicus?
At about 20 weeks; at 12 weeks it is just above the symphysis and at 36 weeks it approaches the costal margin.
What is the difference between term and post-term?
Term spans 37-42 weeks, whereas post-term means 42 completed weeks or beyond.