You do not need to be an... obstetrician, to find out when a baby was made. The gestational age of an embryo/fetus is given in weeks, which start with the first day of the last menstrual period. You get the due birth date by going 40 weeks ahead. Fertilization occurs 10 - 16 days before the next period would have started.
(Note: your country may be calculating these things with a different method or with different numbers and one period cycle could last from 21 days up to 45 days if you’re young)
Example: today is February 11th; if your period starts today, ends a week later and you get pregnant during your next ovulation, then your baby’s calculated birth date would be November 17th (most children are born around that date). The embryo would start existing about a day after ovulation, which would be somewhere at the end February. Because sperm can survive a few days, the day your egg gets fertilized doesn’t need to be the day you last had sex.