Nuchal scan vs dating scan

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The embryo should be seen by the time the gestational sac measures 20mm, about five and a half weeks.

The heartbeat is usually seen on transvaginal ultrasound by the time the embryo measures 5mm, but may not be visible until the embryo reaches 7mm, around 7 weeks gestational age.

This gives an estimate of the weight and size of the fetus and is important when doing serial ultrasounds to monitor fetal growth.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.Current evidence indicates that diagnostic ultrasound is safe for the unborn child, unlike radiographs, which employ ionizing radiation.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.

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