Obstetric dating accuracy
The fallacy in this assumption is that the time of ovulation varies greatly in relation to the menstrual cycle, both from cycle to cycle and individual to individual.
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.Size can be misleading in the presence of multiple pregnancy, uterine fibroids, or a full bladder.Tape measurement of the symphysis-fundus height may be useful up to 28-30 weeks' gestation, beyond which it becomes too inaccurate for dating.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 first 2 are subject to considerable error and should only be used when ultrasonography facilities are not available.