As a general rule, the official gestational age should be based on the actual beginning of the last menstrual period, unless any of the above methods gives an estimated date that differs more than the variability for the method, in which case the difference cannot probably be explained by that variability alone.For example, if there is a gestational age based on the beginning of the last menstrual period of 9.0 weeks, and a first-trimester obstetric ultrasonography gives an estimated gestational age of 10.0 weeks (with a 2 SD variability of ±8% of the estimate thereby giving a variability of ±0.8 weeks), the difference of 1.0 weeks between the tests is larger than the 2 SD variability of the ultrasonography estimate, indicating that the gestational age estimated by ultrasonography should be used as the official gestational age.The popularity of using such a definition of gestational age is that menstrual periods are essentially always noticed, while there is usually a lack of a convenient way to discern when fertilization occurred.The initiation of pregnancy for the calculation of gestational age can be different from definitions of initiation of pregnancy in context of the abortion debate or beginning of human personhood.Obviously, you aren’t pregnant until conception occurs. In many other places, the new knowledge has been incorporated into a dating system that starts with conception.In that system, pregnancy starts on the day of ovulation (usually the day of conception) and lasts for 266 days (38 weeks). It only makes a difference in calculating how far along you are during a pregnancy.This is a holdover from a time when much less was understood about conception.
Check mild interactions to serious contraindications for up to 30 drugs, herbals, and supplements at a time.Pregnancy can be dated from the first day of the last menstrual period or it can be dated from the day of ovulation.In the United States, pregnancy is dated from the first day of the last menstrual period.Measuring the baby using ultrasound is most accurate in early pregnancy. The best time to estimate gestational age using ultrasound is between the 8th and 18th weeks of pregnancy.The most accurate way to determine gestational age is using the first day of the woman’s last menstrual period and confirming this gestational age with the measurement from an ultrasound exam.For a woman with a regular period, conception typically occurs about 11-21 days after the first day of the last period.Most women do not know the exact date of conception because it can be challenging to know exactly when ovulation occurs.However, most women know when their last menstrual period was and it was known that the average pregnancy lasts 280 days (40 weeks) from the first day of the last menstrual period.According to this system, conception doesn’t actually occur until 2 weeks of pregnancy.You can calculate how far along you are using our Pregnancy Calculator.Dating a pregnancy can cause confusion, because it can be done in either of two different ways.Gestational age is calculated from the first day of the mother’s last menstrual period and not from the date of conception.The baby can be measured as early as 5 or 6 weeks after the mother’s last menstrual period.All information contained in and produced by the Med Calc 3000 system is provided for educational purposes only.