Clinical overview
Almost every obstetric decision you will ever make hangs off one number: the gestational age. Whether you offer aspirin prophylaxis, whether a fetus is "growth-restricted" or simply small, whether bleeding at 22 weeks is a previable loss or a delivery, whether to give antenatal corticosteroids, whether "term" induction is safe, how you interpret a CTG, when a baby is "post-dates" and at what threshold you intervene — all of it is anchored to gestational age. Get the dating wrong and you will misclassify growth, mistime delivery, over-investigate normal pregnancies and under-treat abnormal ones. In a South African context, where a large proportion of women book late, do not know their last menstrual period (LMP) reliably, and where first-trimester ultrasound is not universally available, accurate dating is one of the highest-yield skills a registrar develops.
Assessing gestational age is not a matter of reciting that pregnancy lasts 40 weeks; it means choosing the right method for the woman in front of you, applying it correctly, recognising when methods disagree, and acting on the result. The discipline is easy to get wrong: establish the estimated date of delivery (EDD) as early as possible, ideally by first-trimester ultrasound, fix it, and do not move it later in pregnancy without a very good reason. A late-booking woman with no early scan, common in district clinics, forces you to reason from less reliable data and to say so honestly in your documentation.
Core knowledge
Definitions and the convention
Gestational age is counted in completed weeks and days from the first day of the last menstrual period (LMP), not from conception. This is a convention: it assumes a regular 28-day cycle with ovulation and conception around day 14, so gestational age is roughly two weeks more than the true embryonic (post-conceptional) age. A pregnancy is term from 37+0 to 41+6 weeks; preterm is birth before 37+0; post-term is 42+0 weeks or beyond. The EDD by convention is 280 days (40 weeks) from the LMP — Naegele's rule (LMP + 7 days – 3 months + 1 year) is the arithmetic shortcut, though it embeds the 28-day-cycle assumption and is only as good as the recalled LMP.
These week boundaries are not arbitrary trivia; they are decision thresholds. Viability counselling, antenatal corticosteroid windows, magnesium sulphate for neuroprotection, the definition of growth restriction, the timing of "term" elective delivery, and the post-dates surveillance/induction decision all key off them. An error of even one to two weeks can move a fetus across a management boundary.
Why menstrual dating is unreliable
LMP dating fails for predictable reasons, and you should be able to list them: irregular or anovulatory cycles, recent hormonal contraception (especially recent depot medroxyprogesterone acetate, where ovulation may be delayed for months), lactational amenorrhoea, early-pregnancy bleeding mistaken for a period, and simple recall error. Conception does not reliably occur on cycle day 14; in long or irregular cycles it is later, so LMP systematically over-estimates gestation in those women. Standard teaching is that menstrual dating, even when the LMP is "certain," is materially less accurate than a first-trimester scan, and across a population it tends to over-diagnose post-term pregnancy.
Why ultrasound dating works — and its ceiling
Ultrasound dates the pregnancy by measuring how big the embryo or fetus is and reading off a reference chart that maps size to gestational age. It works best early because in the first trimester biological variation in size is small — every normal embryo grows at almost the same rate — so size predicts age tightly. As pregnancy advances, normal fetuses diverge in size (genetics, sex, growth pathology), so a measured biometry could represent a normal fetus of one age or an abnormally grown fetus of another. Ultrasound dating accuracy therefore degrades steadily with advancing gestation, so the earliest adequate scan wins.
The measurements, in order of when they are used:
- Mean sac diameter (MSD) — earliest, before an embryo is visible, least accurate; used to confirm intrauterine pregnancy more than to date.
- Crown–rump length (CRL) — the gold standard for dating. Used roughly from 6–7 weeks until the CRL reaches about 84 mm (around 13–14 weeks). A correctly measured CRL dates to within a few days.
- Biparietal diameter (BPD) and head circumference (HC) — used from the second trimester once the CRL is out of range.
- Femur length (FL) and abdominal circumference (AC) — AC is the most growth-sensitive measurement and the least reliable for dating, but is essential for growth assessment.