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.
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.
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.