Obstetric Dating and Viability Scan
The obstetric dating and viability scan is an early pregnancy ultrasound, usually done between 6 and 13 weeks of gestation. It confirms an intrauterine pregnancy, counts the embryos, determines chorionicity, and establishes viability (an embryonic or foetal heartbeat).
It also estimates gestational age and the expected date of delivery, and excludes early complications (ectopic pregnancy, miscarriage, molar pregnancy).
Transvaginal ultrasound (5 to 9 MHz) is used in the early first trimester because the embryo is small. The gestational sac is seen from 4.5 to 5 weeks, the yolk sac from 5.5 weeks, and the embryo and heartbeat from 6 weeks. Transabdominal ultrasound with a full bladder is used after 8 to 10 weeks.
Between 11 and 13 weeks plus 6 days, the same scan may include the nuchal translucency measurement as part of first trimester screening. Ultrasound uses no ionising radiation, and with usual practice (lowest output, shortest exposure, Doppler avoided in the early embryo) has no proven harm to the pregnancy.
About 1 in 4 to 5 pregnancies end in miscarriage, most in the first trimester, 1 to 2% are ectopic, and about 1 in 80 are twins. Accurate dating matters because it determines the timing of screening tests, the interpretation of foetal growth and the diagnosis of post-term and preterm birth.
The crown rump length is the most accurate dating measure in the first trimester. Accuracy is plus or minus 5 to 7 days between 6 and 13 weeks plus 6 days, and plus or minus 3 days at 7 to 10 weeks. It is more accurate than the last menstrual period when the cycles are irregular or the date is uncertain.
Indications. Confirmation of pregnancy location and viability when there is bleeding, pain, hyperemesis, a history of miscarriage or ectopic pregnancy, previous infertility or assisted conception, or uncertainty about dates. Dating when menstrual dates are uncertain or irregular. Routine early scan to establish gestational age and number of foetuses, and chorionicity.
Suspected multiple pregnancy, molar pregnancy, or uterine anomaly. Pregnancy with an intrauterine device. Discrepancy between uterine size and dates. First trimester screening for chromosomal anomalies (nuchal translucency, nasal bone) at 11 to 13 weeks plus 6 days. Control of chorionic villus sampling. Not as a repeated routine scan without a reason, and not for sex determination.
Contraindications and precautions. None absolute. Transvaginal scanning requires consent and a chaperone, and is safe in pregnancy, including when there is bleeding (the probe does not enter the cervix). A full bladder helps transabdominal views, but an over full bladder can distort the uterus and cause discomfort; the bladder is emptied for transvaginal scans.
Pregnant women with latex allergy need latex free covers. The output power is kept as low as reasonably achievable, with the shortest scan time, and spectral Doppler is avoided before 10 to 12 weeks unless clinically needed. Bad news (non-viable pregnancy) requires sensitive communication, privacy and support.
Preparation and technique. Transabdominal or transvaginal scan with gel and probe cover. The sac is located in the uterus, its position noted (fundal, low), and its mean diameter measured. The yolk sac, embryo and cardiac activity (M-mode shows rate) are documented.
The crown rump length is measured in a true sagittal view, as the longest length of the embryo excluding the limbs and yolk sac, taking the mean of three measurements. The number of embryos, amnions and chorions is determined (the "lambda" or "twin peak" sign for dichorionic and the "T" sign for monochorionic twins). The uterus, adnexa (corpus luteum, masses) and pouch of Douglas are examined.
The nuchal translucency is measured in the mid-sagittal view with a neutral head position, at crown rump length of 45 to 84 mm, by accredited operators.
