Antenatal Ultrasound Scans
Several criteria are measured, including length, abdominal circumference and head circumference.
- Abdominal circumference is used to calculate an estimated birth weight. If there is a defect causing abdominal circumference then weight is vastly over-estimated
- If the head and abdominal circumferences are developing at different rates this can be a sign of problems such as IUGR or defects in the abdominal wall
- Bi-parietal diameter/ head circumference
- Femur/ Humerus length
- Abdominal Circumference
- Head is priority for growth, so in the case of starvation, it will grow normally even if starved.
- Measurement is at umbilicus- Liver covers 90% of this area, so gestational diabetes will mean extra glycogen storage, and a bigger belly.
- Amniotic Fluid Index (AFI) – measures four deepest pools of amniotic fluid in the four quadrants. The normal values range between 10 and 20cm. Excessively low or high values are worrying. Amniotic fluid is mainly produced by urine from the baby. In growth restricted babies blood flow is mainly diverted to the head and brain so not much goes to the kidneys therefore not much urine is produced leading to a low AFI (Oligohydramnios). Conversely, babies with diabetes produce lots of urine so have a very high AFI (Polyhydramnios).
- Amniotic fluid is not only a good indicator of the baby’s health, but plays a vital role in the baby's development of the respiratory system: the swallowing of the amniotic fluid helps mature the airways and lungs.
- Doppler scans look for notching of umbilical arteries, which cause high resistance leading to a low flow environment. It is caused by the elastin that is normally removed from the arterial wall at 23wks remaining. The notching therefore leads to small babies.
Based on fetal:
- Amniotic fluid volume
- Breathing rate
- Heart Rate