First trimester dating ultrasound

Yolk sacs larger than 6 mm are usually indicative of an abnormal pregnancy.Failure to identify (with transvaginal ultrasound) a yolk sac when the gestational sac has grown to 12 mm is also usually indicative of a failed pregnancy.

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Thus, for these early pregnancies, the actual cardiac rate is less important that its presence or absence.

Sometimes, with normal pregnancies, the fetal heartbeat is not visible until a fetal pole of up to 4 mm in length is seen.

Failure to identify fetal cardiac activity in a fetus whose overall length is greater than 4 mm is an ominous sign.

It can sometimes be difficult identifying a fetal heartbeat from the background movement and maternal pulsations.

First trimester scanning is useful to identify abnormalities in the early development of a pregnancy, including miscarriage and ectopic pregnancy, and provides the most accurate dating of a pregnancy.

Technique First trimester scanning can be performed using either an abdominal approach or a vaginal approach.

Gestational Sac The gestational sac is the earliest sonographic finding in pregnancy.

The gestational sac appears as an echogenic (bright echoes) ring surrounding a sonolucent (clear) center.

This is most easily accomplished by first identifying the internal iliac vessels.

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