No we can’t. When we count the gestational age, we usually start counting in relation to the previous menstrual period. If the mentrual cycle is 28 days, then ovulation happens 2 weeks after the period, and you can only get pregnant at around that time, but the pregnancy test won’t be positive yet because not enough pregnancy hormones have built up. You usually do your pregnancy test after you’ve missed your period to get an accurate result, so by the time you can find out whether you’re pregnant, you’ll be at least 4-5 weeks pregnant (if the test is positive) according to menstrual dates. Subsequently, when you go for a dating scan (more accurate), the gestational age may change again. Sometimes it’s close to what you counted based on your period date, and sometimes it can be 1-2 weeks of difference.