An ultrasound can help estimate this if your last menstrual period is unknown.
A doubling of the level of beta-h CG over a period of about 48 hours is considered normal and normally excludes the diagnosis of ectopic pregnancy.
You can calculate your due date by subtracting three months from the first day of your last menstrual period (LMP) and then add a week. A pregnancy is based on being 280 days long, which is 40 weeks (more like 10 months not 9! When we give you a due date we consider 37 to 42 weeks to be full-term, so even if your baby is born two weeks before your due date it is not considered premature.
The accuracy of using your LMP to date your pregnancy depends on having regular cycles and assumes you ovulated (released an egg) on day 14 of your cycle and conceived right away - none of which may be true.
A first trimester ultrasound often requires a higher resolution due to the small size of the fetus. yolk sac is a structure that provides nourishment to the fetus while the placenta is developing.
Endovaginal ultrasound examination is another option. By about six weeks, an ultrasound can show other things as well.