Early ultrasound gestational age dating
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.
Other algorithms take into account other variables, such as whether this is the first or subsequent child, the mother's race, age, length of menstrual cycle, and menstrual regularity.
In order to have a standard reference point, the normal pregnancy duration is assumed by medical professionals to be 280 days (or 40 weeks) of gestational age.
There is also considerable variability in this interval, with a 95% prediction interval of the ovulation of 9 to 20 days after menstruation even for an average woman who has a mean LMP-to-ovulation time of 14.6.
The actual variability between gestational age as estimated from the beginning of the last menstrual period (without the use of any additional method mentioned in previous section) is substantially larger because of uncertainty which menstrual cycle gave rise to the pregnancy.
Also, vaginal bleeding occurs during 15-25% of first trimester pregnancies, The mean pregnancy length has been estimated to be 283.4 days of gestational age as timed from the first day of the last menstrual period and 280.6 days when retrospectively estimated by obstetric ultrasound measurement of the fetal biparietal diameter (BPD) in the second trimester.
The fertilization age (also called embryonic age and later fetal age) is the time from the fertilization.
It usually occurs within a day of ovulation, which, in turn, occurs on average 14.6 days after the beginning of the preceding menstruation (LMP).
Babies who were less than 28 weeks of gestational age, or weighed less than 1000 grams, or less than 35 cm in length – even if they showed some sign of life (breathing, heartbeat, voluntary muscle movement) – were classified as "live fetuses" rather than "live births." Only if such newborns survived seven days (168 hours) were they then classified as live births.
If, however, they died within that interval, they were classified as stillbirths.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.Gestational age is a measure of the age of a pregnancy which is taken from the woman's last menstrual period (LMP), or the corresponding age of the gestation as estimated by a more accurate method if available.