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.

Yolk Sac

Yolk Sac

6 Week Intrauterine Pregnancy

6 Week Intrauterine Pregnancy

Embryonic Heart Beat

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.

8 Week Embryo

8 Week Embryo

10 Week Embryo

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.

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