Ultrasound scan basics for clinicians
Practical advice about pregnancy scans
Feedback from the frontline
One of the critical roles the clinicians play in maternity care is in the information given to women about recommended testing and, having obtained informed consent, appropriate referrals for blood, urine and ultrasound testing. There are so many options and pressure to obtain instant answers. How can clinicians best advise women of their options, including the Medicare funding of scans?
The following advice is general only, for more detailed advice about a particular case, please seek input from your obstetric and/or radiological colleagues.
The National Antenatal Guidelines recommend obtaining an early dating scan in women who are unsure about their dates. It is, however, clearly possible to scan too early (4-5 weeks) resulting in there being uncertainty of even an intrauterine pregnancy. As a rule of thumb, scans done at an estimated from 8 weeks’ gestation are a safe bet, as the fetal heart can usually be reliably seen from 6 weeks, giving a couple of weeks leeway should the dates be out. If the woman is uncertain of her dates, a BHCG level can be used as a guide to when a scan might be useful, recognising that it is quite unreliable for actual dating. 98% of singleton pregnancies will have an identifiable intrauterine gestation sac once the BHCG exceeds 1500, and if going by good dates, it is usually visible between 5-6 weeks on a transvaginal scan (TVS). Allowing for the usual doubling of BHCG in an ongoing pregnancy, once you know the level, you can advise the woman on the appropriate timing for her scan. For example, if the level is 1500, waiting a week before the woman has a scan should allow the pregnancy to be advanced enough to give adequate information on dating and to confirm viability. Obviously, if this is a multiple pregnancy, these levels will not be helpful. Just to make things difficult, around 15% of ongoing intrauterine pregnancies may not double appropriately. Please let women know that, for accuracy, most early dating scans performed <8 weeks’ gestation will involve a transvaginal element.
If we are concerned a woman may have an ectopic pregnancy (classic triad = 6-8 weeks amenorrhoea, bleeding and pain) but there is no intrauterine gestation sac, we can use the BHCG to guide us. Once the BHCG level is above 1500, an intrauterine gestation sac should be seen and if not, the woman should be considered at risk of an ectopic until proven otherwise. Sometimes an ectopic pregnancy will be visible, if not it may be deemed a pregnancy of unknown location (PUL). Be aware that in around 15% of ectopic pregnancies, the BHCG may still double every 48hrs. If it is climbing slowly, we will be more suspicious about the possibility of an ectopic and hence would watch more closely and liaise early with our obstetric colleagues. Please let women know a transvaginal scan is usually done in these cases.
Nuchal Translucency Scan (NTS)
In the era of the non-invasive prenatal test (NIPT), the NTS remains an important part of the recommended screening process. We are now able to test women for chromosomal, anatomical and the parents for risk of recessive genetic mutations (e.g. cystic fibrosis, fragile X syndrome and spinal muscular atrophy) and clearly, there are different tests we use for these aspects of screening. CVS or amniocentesis will identify chromosomal or genetic anomalies, but not anatomical ones. NTS can identify a limited range of major structural anatomical abnormalities, allowing the earlier diagnosis of important conditions including anencephaly and abdominal wall defects (omphalocele and gastroschisis). The NTS, especially when combined with BHCG and PAPP-A as the first trimester combined screen (FTCS), gives us information about the risk of chromosomal anomalies such as T21, T13 and T18. Technically, these are best done at 12-13 weeks gestation, as the anatomical detail is better seen, however should further testing be recommended, the timeframes for action are then tighter. A transvaginal scan may sometimes be required. The NTS is also helpful to confirm dates and viability and to assess chorionicity in multiples.
Non-invasive Prenatal Testing (NIPT) uses placental DNA in the maternal blood to assess risk for the common trisomies and sex chromosomes. Some NIPT providers also report additional information about possible chromosomal abnormalities e.g. trisomies associated with miscarriage. NIPT is more accurate in the detection of the common trisomies and it also has a lower screen positive rate than the FTCS, but it is still a screening test, with CVS and Amniocentesis the recommended diagnostic tests for women with high risk results.
Feedback from ultrasound practices is that women may be incorrectly booked by reception staff for the equivalent of a dating scan if “12-week anatomy scan” is requested, rather than NTS. They request we continue to order the NTS so that they can ensure the woman is booked into the appropriate time slot with an appropriately trained sonographer and to allow them to apply the correct Medicare code.
The morphology scan (18+0 to 20+6), remains the mainstay of antenatal screening for fetal structural abnormalities. The morphology scan provides additional information about CNS, cardiac, thoracic, skeletal and renal anomalies however some abnormalities may not be detected until later in pregnancy e.g. duodenal atresia, achondroplasia. Like the NTS, it is not 100% accurate and, in most circumstances, the later the scan i.e. 20 vs 18 weeks the more detail is seen. It reliably detects T13 and T18, but performs poorly as a screening tool for T21 and should not be used as such. It also provides information about the position of the placenta which, if low-lying, should be checked again at 34-36 weeks gestation. In women at risk of preterm birth or with a history of cervical surgery (Cone biopsy, LLETZ etc.) it is recommended that a transvaginal scan for cervical length is also requested.
Ultrasound for fetal size or growth
A single scan provides information about fetal size but not about growth velocity/trajectory. Hence when ordering a scan for suspected small for gestational age (SGA) or large for gestational age (LGA) the results can be useful for planning further imaging or delivery. If there is suspected SGA or reduced growth velocity clinically it may be necessary to have more than one scan to evaluate change in fetal growth over time.
Women are entitled to receive a Medicare rebate so long as they meet the eligibility criteria, which includes uncertain dates, threatened miscarriage, ? ectopic, advanced maternal age and, for NTS in particular, risk of fetal anomaly. Please write the appropriate criteria on the request form so that our imaging colleagues can use the correct Medicare item and bill appropriately. For most women, there will be an out of pocket cost for scans which varies but in South Brisbane, typically sits ~ $100-170 above the rebate.
What do you do with what you find?
Obviously, should an anomaly be identified, prompt referral to the obstetric team is warranted and they may need to liaise with Maternal Fetal Medicine and geneticists, +/- surgeons/cardiologists/urologists/paediatricians. The woman will need practical and psychological support, so do be aware of peer, professional and NGO support in your local area.
With thanks and acknowledgment to Dr Kerry McMahon and Dr Glenn Gardener