3 doctors weighed in:

How does a doctor calculation the number of weeks a woman is pregnant?

3 doctors weighed in
Dr. Ted Anderson
Gynecology
1 doctor agrees

In brief: Multiple ways

There are a few different ways to calculate this.
It can be counted at the number of completed weeks after your last menstrual cycle. It can be calculated by the ultrasound measurement of the growing fetus. Once the due date is known (from one of the two methods above), it can be calculated by working backwards from the known due date using a device called a "pregnancy wheel".

In brief: Multiple ways

There are a few different ways to calculate this.
It can be counted at the number of completed weeks after your last menstrual cycle. It can be calculated by the ultrasound measurement of the growing fetus. Once the due date is known (from one of the two methods above), it can be calculated by working backwards from the known due date using a device called a "pregnancy wheel".
Dr. Ted Anderson
Dr. Ted Anderson
Thank
Dr. Stephen Sudler
Obstetrics & Gynecology

In brief: Based on LMP

Thje gestational age of a pregnancy is based on the number of weeks from the start of the last normal menstrual period.
This also assumes a menstrual cycle of 28 days. The lmp was supposedly chosen as the signficant event a woman could remember, and the date of conception could be variable (and perhaps not very memorable, sorry guys). Or (-3 months + 7 days) works as well for the due date.

In brief: Based on LMP

Thje gestational age of a pregnancy is based on the number of weeks from the start of the last normal menstrual period.
This also assumes a menstrual cycle of 28 days. The lmp was supposedly chosen as the signficant event a woman could remember, and the date of conception could be variable (and perhaps not very memorable, sorry guys). Or (-3 months + 7 days) works as well for the due date.
Dr. Stephen Sudler
Dr. Stephen Sudler
Thank
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