A positive fetal heartbeat at this stage indicates a 95 percent chance that the pregnancy will proceed normally, according to Dr. Early transvaginal ultrasound, the most reliable way to determine a pregnancy due date, can date a pregnancy within a few days and is more accurate than determining a due date by the date of the last menstrual period.

Before the advent of routine ultrasound in pregnancy, medical personnel estimated a baby's due date by the date of the last menstrual period and by feeling the uterus and determining the size to see if it matched the given due date.

With transvaginal ultrasound, checking for embryonic development known to occur within a certain time frame more accurately dates a pregnancy.

It is not uncommon for babies that are labeled “Large for Gestational Age (LGA)” and “Intra Uterine Growth Restriction (IUGR)” to have monthly or even weekly ultrasounds during the pregnancy.

When an ultrasound is performed, measurements of the head, abdomen, thigh, and amount of amniotic fluid are done.

Pregnancy is dated from the first day of the last menstrual period, so at four weeks, a woman is just due for a menstrual period.

At 4.5 to 5 weeks, a gestational sac may be seen in the uterus using transvaginal ultrasound, Joseph Woo, M.

In other words, by comparing your baby’s measurements to the data from this large collection of measurements, the ultrasound can then tell how far along your baby is.

However, if you have a larger than average baby, the ultrasound will apply the husky figures to the “normal” measurements.

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).