4 doctors weighed in:

I had a real quick ultrasound at 11 weeks (one minute).I am sure of the date of conception, so I know I am 11 weeks. The ultrasound dated me 12 weeks.?

4 doctors weighed in
Dr. Gerard Honore
Fertility Medicine
2 doctors agree

In brief: Dating by LMP

Dating is from the first day of the menstrual period not date of conception, so make sure you aren't comparing apples and oranges.
The accuracy of us dating in the first trimester is also considered to be +/- 3-4 days. Finally, by convention, if us and menstrual dates agree to within 1 week, we use the menstrual dates. Does this help?

In brief: Dating by LMP

Dating is from the first day of the menstrual period not date of conception, so make sure you aren't comparing apples and oranges.
The accuracy of us dating in the first trimester is also considered to be +/- 3-4 days. Finally, by convention, if us and menstrual dates agree to within 1 week, we use the menstrual dates. Does this help?
Dr. Gerard Honore
Dr. Gerard Honore
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Marianne DiNapoli
Obstetrics & Gynecology

In brief: Ovulation varies

Ovulation, when the egg is released to be fertilized, usually occurs two weeks after your last period.
However, this can vary from woman to woman, and even from month to month in the same woman. The ultrasound at 11 weeks may be more accurate than dating the pregnancy by the date of your last period, if the dates were off by more than 1 week.

In brief: Ovulation varies

Ovulation, when the egg is released to be fertilized, usually occurs two weeks after your last period.
However, this can vary from woman to woman, and even from month to month in the same woman. The ultrasound at 11 weeks may be more accurate than dating the pregnancy by the date of your last period, if the dates were off by more than 1 week.
Marianne DiNapoli
Marianne DiNapoli
Answer assisted by Marianne Di Napoli, Medical Student
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