Obstetric dating scan

Gestational age (GA) refers to the length of pregnancy after the first day of the last menstrual period (LMP) and is usually expressed in weeks and days. Conceptional age (CA) is the true fetal age and refers to the length of pregnancy from the time of conception.

This terminology does vary geographically and over time, and it may need clarification if not explicitly defined in relevant articles.

The accuracy of menstrual history in women with a history of oligo-ovulation, such as those with polycystic ovarian syndrome, should be questioned.

If conception occurred while oral contraceptives or long-acting progestogens were being taken, the LMP cannot be used because it has no relation to the time of ovulation.

Pregnant women should be counselled that only 4% of all babies are born precisely on the estimated date of confinement.

Failure to appreciate this may lead to unnecessary maternal anxiety if a pregnancy progresses beyond the EDD.

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The date of feeling the first fetal movements (quickening) is far too unreliable to be useful.

Therefore, giving a range for the likely date of birth (eg, EDD -2 weeks to EDD 1 week) is more useful.

Infants born before 37 completed weeks' gestation are deemed preterm, whereas those born after 42 weeks' are considered post-term.

The 95% confidence interval of menstrual dates is -27 to 9 days.

To further complicate matters, 10-45% of pregnant women cannot provide useful information about their LMP, and 18% of women with certain menstrual dates have significant differences between menstrual and ultrasonographic dating.