Early ultrasound gestational age dating
Nuchal translucency scan This is a scan which is usually carried out at 11-13 weeks and is a method of screening for chromosomal abnormalities without having an invasive test such as an amniocentesis.
The scan can confirm the number of babies in the uterus, the embryo can be observed and measured by about five and a half weeks and a heartbeat usually detected by 6 weeks.
A balance must always be maintained between diagnostic benefit and risk to the patient.
The position of BMUS on the use of ultrasound to produce “keepsake” images of the fetus is outlined in the European Committee of Medical Ultrasound Safety (ECMUS) statement, endorsed by BMUS. Ultrasound scans should not be performed solely for producing souvenir images or recordings of a fetus or embryo. The production of souvenir images or recordings for the parents to keep is reasonable if they are produced during a diagnostic scan, provided that this does not require the ultrasound exposure to be greater in time or magnitude (as indicated by the displayed MI and TI) than that necessary to produce the required diagnostic information. Attention is drawn to the recommendation of the EFSUMB Clinical Safety Statement for Diagnostic Ultrasound that ultrasound examinations should be performed only by competent personnel who are trained and updated in ultrasound safety matters.
Dating is more accurate when done early in the pregnancy.
When a later scan gives a different estimate of gestational age, the estimated age is not normally changed but rather it is assumed the fetus is not growing at the expected rate.