If you understand some basic facts of conception, your dilemma about the day of conception would be clear.
The conception takes place on or within 24 hours of the egg release (ovulation) as the egg can live only up to 24 hours.
The day of ovulation is 14 days prior to the next expected menses. You had your last menstrual period on 21 January and the next expected period was 25 February (21 Feb + 4 days). So your day of ovulation is about 11 February. Therefore the day of conception would be around that day.
Though clinically the period of gestation is calculated from the date of last menstrual period (LMP), actual conception takes place around 2 weeks later. This has probably created the confusion for you. Pregnancy
of 3-4 weeks from the date of LMP is actually 2 weeks lesser in age.
To further clarify this dilemma, you may get ultrasonography
done. By ultrasonography, the age of the embryo is calculated by its size. Hence it would be the real age of the embryo; i.e. calculated from the day of conception. However, please be aware that in such early stages of pregnancy, there could be variation by few days to a week in calculating the age of embryo.
I hope this resolves your dilemma.
Dr. Nishikant Shrotri