Additionally, the ISUOG recommends that pregnant women have obstetric ultrasounds between 11 weeks' and 13 weeks 6 days' gestational age in countries with resources to perform them (the nucal scan).Performing an ultrasound at this early stage of pregnancy can more accurately confirm the timing of the pregnancy and can also assess for multiple fetuses and major congenital abnormalities at an earlier stage.
A gestational sac can be reliably seen on transvaginal ultrasound by 5 weeks' gestational age (approximately 3 weeks after ovulation).
Transvaginal scans usually provide clearer pictures during early pregnancy and in obese women.
Also used is Doppler sonography which detects the heartbeat of the fetus.
The embryo should be seen by the time the gestational sac measures 20 mm, about five-and-a-half weeks.
The heartbeat is usually seen on transvaginal ultrasound by the time the embryo measures 5 mm, but may not be visible until the embryo reaches 7 mm, around 7 weeks' gestational age.
Obstetric sonography has become useful in the assessment of the cervix in women at risk for premature birth.