Although the dating of pregnancy accuracy takes a dive, still ultrasound is indispensable for the other uses.
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.
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.
Once the fetal pole is seen, the crown-rump length (CRL) of the embryo can be measured. Once the CRL reaches 5 millimeters, the heartbeat should appear, the APA states.
This occurs normally between 6 and 7 weeks of pregnancy.
By 6 to 7 weeks, it’s usually possibly to see the fetal pole, the earlier sign of the developing embryo, within the gestational sac on transvaginal ultrasound.
If the gestational sac reaches 16 to 18 millimeters and no fetal pole is seen, the pregnancy may be abnormal, according to the APA. Later measurements that don’t reflect the date given by an early CRL means the pregnancy isn’t growing normally, not that the early CRL was wrong.
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.
D., explains in "Obstetric Ultrasound: A Comprehensive Guide".