A first trimester dating scan is a fundamental part of antenatal care and should be made available to all women attending maternity units nationwide, a new study has concluded.

According to researchers from Cork University Maternity Hospital and University College Cork, obstetric ultrasound “is a core part of modern antenatal care” and dating scans should ideally be performed between 10 and 14 weeks gestation.

“The importance of dating scans includes the confirmation of viability, the accurate determination of gestational age, the identification of multiple pregnancy and foetal/maternal structural anomalies.

“These factors are fundamental to determining a woman’s pathway of care and stratifying her risk category, a key component of the Irish National Maternity Strategy 2016,” they explained.

They noted that accurate determination of gestational age has a key role in reducing the incidence of unnecessary induction of labour for suspected prolonged  pregnancy “and is crucial in the extremes of viability where discrepancy of a few days can significantly affect decision making”.

First trimester ultrasound is also critical when it comes to planning appropriate antenatal management in the case of a multiple pregnancy. It also helps to facilitate parental choice with respect to continuing or terminating a pregnancy in the case of a major foetal abnormality.

However, there is a wide variation in the provision of obstetric ultrasound in Ireland for a number of reasons, including the absence of national ultrasound guidelines and “the suboptimal number of trained sonographers in many units”, the researchers noted.

They set out to illustrate the benefits of a structured dating ultrasound programme in a tertiary setting by carrying out a retrospective review of dating scans performed between January and December 2017. A total of 6,077 dating ultrasound scans were reviewed.

Among those attending for a first dating ultrasound, 82% were measured between 10 and 14 weeks.

When it came to last menstrual period, 16% of women relied on the dating ultrasound scans as the primary source for estimation of gestational age and estimated due date.  This was due to a number of reasons such as being unable to recall the last menstrual period or having an irregular cycle.

Where last menstrual period was recorded, ultrasound dating estimates were compared and the mean difference between the two was 3.9 days.

Meanwhile, 98% of pregnancies were found to be viable, while 1.5% were non-viable. In total, 94 missed miscarriages were recorded.

Almost 2% of pregnancies were multiple – 110 twin pregnancies and two triplet pregnancies.

Foetal abnormalities were detected in 0.4% of scans, with increased nuchal translucency being the main anomaly detected (68%).

Uterine anomalies were also detected in 1.2% of scans, with uterine fibroids being the most frequent anomaly seen (88%).

The researchers noted that in 2017, only 47% of women were offered a first trimester ultrasound scan performed by a suitably qualified sonographer nationally. This, they pointed out, does not meet the fundamental aims of the 2016 National Maternity Strategy.

“Other studies have highlighted the lack of a national guideline for obstetric ultrasound and the variation in providing the service between the maternity units in Ireland. This shows that there is a lot more to do to improve and standardise antenatal care for women nationally,” they said.

They added that this study reinforces the need for routine scanning of all pregnancies in the first trimester.

“Research and policy indicate that it is a fundamental part of antenatal care and should be made available to women attending all maternity units nationally.”

This study is published in the Irish Medical Journal.