Accuracy of pregnancy dating ultrasound
With transvaginal ultrasound, checking for embryonic development known to occur within a certain time frame more accurately dates a pregnancy.
Transvaginal ultrasound can see embryonic development about a week before transabdominal ultrasound, the American Pregnancy Association (APA) states.
When an ultrasound is performed, measurements of the head, abdomen, thigh, and amount of amniotic fluid are done.
These measurements are computed automatically in the ultrasound machine’s software.
The software has certain measurement scales based on data from large populations, and your baby’s measurements are put into this scale.
Of all of these uses, dating the pregnancy is the most common reason to use ultrasound, particularly when the expectant mother cannot remember the date of her last period (as in breast-feeding or irregular cycles).
Even when the last period is known, ultrasound is reassuring to demonstrate adequate growth, especially when there’s a risk of delayed growth, as in hypertension or smoking, or if there’s the risk of exaggerated growth, as in gestational diabetes.
The length of your pregnancy is calculated from the first day of your last menstrual period (LMP), even though you don't actually conceive until 14 days after your LMP, and later than this if your cycle is longer than 28 days.