ultrasound in the diagnosis of pregnancy

Last reviewed 01/2018

At about five weeks from the last menstrual period, a gestational sac is visible by ultrasound. A week later an embryo may be observed within the sac.

At six weeks the heart pulsations are visible and a crown- rump length may be measured.

At twelve weeks the placenta and its site of insertion are clearly visible.

Using ultrasound for diagnosis of pregnancy

• if a woman attends an early pregnancy assessment service (or out-ofhours gynaecology service if the early pregnancy assessment service is not available) then a transvaginal ultrasound scan should be offered to identify the location of the pregnancy and whether there is a fetal pole and heartbeat
• a transabdominal ultrasound scan should be considered for women with an enlarged uterus or other pelvic pathology, such as fibroids or an ovarian cyst
  • also offer a transabdominal ultrasound scan if a transvaginal ultrasound scan is unacceptable to the woman
• the diagnosis of miscarriage using an ultrasound scan cannot be guaranteed to be 100% accurate and there is a small chance that the diagnosis may be incorrect, particularly at very early gestational ages

Determining viability of intrauterine pregnancy

• checklist when performing an ultrasound scan to determine the viability of an intrauterine pregnancy
  • first look to identify a fetal heartbeat. If there is no visible heartbeat but there is a visible fetal pole, measure the crown-rump length
  • only measure the mean gestational sac diameter if the fetal pole is not visible
    
    
    • crown-rump length
      • if the crown-rump length is less than 7.0 mm with a transvaginal ultrasound scan and there is no visible heartbeat, perform a second scan a minimum of 7 days after the first before making a diagnosis. Further scans may be needed before a diagnosis can be made
      • if the crown-rump length is 7.0 mm or more with a transvaginal ultrasound scan and there is no visible heartbeat:
        • seek a second opinion on the viability of the pregnancy and/or
        • perform a second scan a minimum of 7 days after the first before making a diagnosis
      • if there is no visible heartbeat when the crown-rump length is measured using a transabdominal ultrasound scan:
        • record the size of the crown-rump length and
        • perform a second scan a minimum of 14 days after the first before making a diagnosis
          
          
    • gestational sac diameter
      • if the mean gestational sac diameter is less than 25.0 mm with a transvaginal ultrasound scan and there is no visible fetal pole, perform a second scan a minimum of 7 days after the first before making a diagnosis. Further scans may be needed before a diagnosis can be made
      • if the mean gestational sac diameter is 25.0 mm or more using a transvaginal ultrasound scan and there is no visible fetal pole:
        • seek a second opinion on the viability of the pregnancy and/or
        • perform a second scan a minimum of 7 days after the first before making a diagnosis
          
          
    • no visable and the mean gestational sac diameter is measured using a transabdominal ultrasound scan:
      
      • record the size of the mean gestational sac diameter and perform a second scan a minimum of 14 days after the first before making a diagnosis

Notes:

• do not use gestational age from the last menstrual period alone to determine whether a fetal heartbeat should be visible
• when diagnosing complete miscarriage on an ultrasound scan, in the absence of a previous scan confirming an intrauterine pregnancy, always be aware of the possibility of ectopic pregnancy. Advise these women to return for further review if their symptoms persist

Reference:

• 1) NICE (December 2012).Ectopic pregnancy and miscarriage