Many obstetricians date the onset of pregnancy from the date:

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Obstetricians typically date the onset of pregnancy from the woman's last menstrual period. This is because it is easier to track and predict the pregnancy milestones such as due date, fetal development, and trimesters based on a regular menstrual cycle. While conception and implantation are important milestones in pregnancy, they are not as easily dated or predictable as the last menstrual period. Similarly, the date of intercourse may not necessarily coincide with conception, making it an unreliable marker for dating pregnancy.

Many Obstetricians Date the Onset of Pregnancy from the Date