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The International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) recommends that pregnant women have routine obstetric ultrasounds between 18 weeks' and 22 weeks' gestational age (the anatomy scan) in order to confirm pregnancy timing, to measure the fetus so that growth abnormalities can be recognized quickly later in pregnancy, and to assess for congenital malformations and multiple pregnancies (twins, etc).
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).
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.
Gestational age is usually determined by the date of the woman's last menstrual period, and assuming ovulation occurred on day fourteen of the menstrual cycle.
This gives an estimate of the weight and size of the fetus and is important when doing serial ultrasounds to monitor fetal growth.
Obstetric sonography has become useful in the assessment of the cervix in women at risk for premature birth.
A gestational sac can be reliably seen on transvaginal ultrasound by 5 weeks' gestational age (approximately 3 weeks after ovulation).
The embryo should be seen by the time the gestational sac measures 20 mm, about five-and-a-half weeks.
Some abnormalities detected by ultrasound can be addressed by medical treatment in utero or by perinatal care, though indications of other abnormalities can lead to a decision regarding abortion.
A short cervix preterm is undesirable: At 24 weeks' gestation a cervix length of less than 25 mm defines a risk group for preterm birth, further, the shorter the cervix the greater the risk.
In most countries, routine pregnancy sonographic scans are performed to detect developmental defects before birth.
Randomized controlled trials have followed children up to ages 8–9, with no significant differences in vision, hearing, school performance, dyslexia, or speech and neurologic development by exposure to ultrasound.
Several randomized controlled trials have reported no association between Doppler exposure and birth weight, Apgar scores, and perinatal mortality.
Some resources indicate that there are clear reasons for this and that such scans are also clearly beneficial because ultrasound enables clear clinical advantages for assessing the developing fetus in terms of morphology, bone shape, skeletal features, fetal heart function, volume evaluation, fetal lung maturity, Second-trimester ultrasound screening for aneuploi- dies is based on looking for soft markers and some predefined structural abnormalities.