First Trimester Ultrasound


Most pregnant women will be offered a routine first trimester ultrasound examination to assess the early fetal anatomy. It is usually performed between 12 and 14 weeks gestation, and ideally at 13 weeks. The ultrasound is performed transabdominally. A full bladder is not required.

The purpose of this ultrasound is:

  • To confirm that the fetus is alive.

  • To diagnose multiple pregnancy if no earlier scan was performed

  • To confirm your due date if no earlier scan was performed

  • To assess fetal growth by measuring the fetal head, abdomen and femur

  • To assess the fetal anatomy. This involves a detailed early ultrasound examination of the fetal head, brain, face, heart, stomach, lungs, abdominal wall, kidneys, bladder, spine, arms, legs, hands, and feet.

  • To assess the amniotic fluid volume.

  • To assess the length of the cervix.

  • To assess the mothers ovaries for cysts

  • To assess the uterus for lesions such as fibroids.

This examination has the ability to detect a large number of major fetal malformations. However it is important to be aware that even with the best ultrasound equipment not all abnormalities can be seen in the first trimester ultrasound. The majority of major malformations may be detected at the subsequent mid trimester fetal anatomy ultrasound performed at 20 weeks (for further information please follow link to “Routine 20 week ultrasound”)

Sometimes abnormalities can be missed because suboptimal views are obtained. This can be due to difficult fetal position or the patient's weight. In these situations a transvaginal ultrasound may be advised by the doctor performing the scan. If the views remain suboptimal, a repeat ultrasound may be recommended (usually between 15 to 17 weeks gestation) when the fetus is bigger and/or in a different, hopefully more favourable position.

Other times abnormalities may be missed despite good views. This may happen with conditions where the abnormality only becomes evident in later pregnancy or where there are in fact no structural changes in the fetus.