How to calculate age at conception
Calculating Conception. Gestational age, or the age of the baby, is calculated from the first day of the motherâs last menstrual period.Since the exact date of conception is almost never known, the first day of the last menstrual period is used to measure how old the baby is. Use our Pregnancy Calculator to calculate these dates for you.he baby can be measured as early as 5 or 6 weeks after the motherâs last menstrual period. Measuring the baby using ultrasound is most accurate in early pregnancy. It becomes less accurate later in pregnancy. The best time to estimate gestational age using ultrasound is between the 8th and 18th weeks of pregnancy.