The age of the developing fetus, gestational age, can be calculated from the first day of the mother’s last menstrual period. Calculating pregnancy due date can’t be done very accurately, but the time of the last menstrual period can be used to measure how old the baby is. Gestational age can be calculated if the mother has regular periods and knows the first day of her last menstrual period. The alternate method for calculating pregnancy due date is ultrasound scanning. In ultrasound scanning the baby’s age can be measured as early as five or six weeks after the mother last menstrual period. Calculating pregnancy due date using ultrasound gives the most accurate result during early pregnancy. Calculating pregnancy due date in between eight and eighteen weeks of pregnancy gives the accurate gestational age of baby.
During a typical pregnancy, for a lady with regular menstrual periods, conception typically occurs about eleven to twenty one days after the first day of the last period. Most women do not know the exact date of their period and their conception date is merely an estimate based on the first day of their last period. Women those undergo special procedure such as artificial insemination or in vitro fertilization typically know the accurate date of conception. Normally, calculate pregnancy due date is estimated after forty weeks from the last day of the period. This is just an estimate, normally five percent of babies are born on their estimated due date. But there are bound to be some difficulties in determining the gestational age of baby if the women have irregular menstrual period or women who cannot remember the first day of their last menstrual period, it can be difficult to determine gestational age using this method. In such cases, an ultrasound scanning is often required to determine the gestational age. There are some cases in which determining the gestational age is difficult because of unusual large size of a small baby.