Early Pregnancy Assessment and Dating

What information is gained from an early pregnancy scan?

• The location of the pregnancy
• The number of embryos in the uterus
• The presence of a heart beat in the embryo
• The length of the embryo (this will establish the dating of the pregnancy)
• The presence of uterine fibroids or ovarian cysts in the pelvis

Why might an ultrasound be required in the first 11 weeks of pregnancy?

• Abdominal pain
• Vaginal bleeding
• Uncertain menstrual dates
• Previous recurrent miscarriage
• Previous ectopic pregnancy
• Women at risk of multiple pregnancies (eg pregnancy following ovulation induction therapies)

What can be seen in early pregnancy?

In general, the pregnancy sac is visible with ultrasound at about 5 weeks gestation (this is calculated from the last menstrual period, not conception). Usually a transvaginal assessment is needed as scanning through the abdomen does not provide enough detail at this very early stage of pregnancy.

By 5-6 weeks gestation the yolk sac is visible, although the embryo is usually not reliably seen until about 6 weeks gestation when it measures approximately 5mm in length. The embryonic heart beat should be visible at 6 weeks gestation.

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  Yolk Sac
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  6 Week Intrauterine Pregnancy
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  Embryonic Heart Beat

By 8 weeks gestation the embryo can usually be seen readily with abdominal scanning. The limbs of the embryo are visible at 9-10 weeks gestation with body movements observed.

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  8 Week Embryo
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  10 Week Embryo

How accurate is pregnancy dating?

A dating ultrasound of the length of the embryo when performed at 7-11 weeks gestation is accurate to within about 3 days. Beyond this time the accuracy reduces due to flexion (bending) of the fetus and individual growth variance.