First-trimester ultrasound

Routine ultrasound should not be offered or requested simply to confirm an ongoing early pregnancy in the absence of any clinical concerns, symptoms or specific indications.

The first ultrasound of the pregnancy should ideally be offered when the gestational age is thought to be between 12 and 13+6 weeks gestation for optimal assessment of fetal anatomy and nuchal translucency (NT). The purpose of the scan is to confirm viability, accurately establish gestational age, determine the number of viable fetuses, evaluate gross fetal anatomy and, if requested, assess the NT as part of the risk assessment for aneuploidy.

Indications for early pregnancy scans (less than 12 weeks)

Indications include:

  • bleeding or pain in early pregnancy, or concern about pregnancy loss (section 88 codes TA and EP)[1]
  • consideration of termination of pregnancy (section 88 code CT)
  • unknown dates* (section 88 code BA)
  • hyperemesis gravidarum
  • trauma
  • pregnancy with an intrauterine contraceptive device (IUCD) in situ
  • previous ectopic pregnancy (section 88 code EP)
  • complex medical conditions where a change of medication may be indicated such as warfarin.

*   Please note: Confirmation of dates by ultrasound is not routinely required before the 12-week scan.

[1] See Appendix 1: Ultrasound scan codes and indications for a full list of the current codes for claiming for ultrasound scans through the New Zealand Public Health and Disability Act 2000 Primary Maternity Services Notice 2007.

Required clinical details

  • Last menstrual period (LMP)
  • Woman’s symptoms
  • Beta human chorionic gonadotropin (βhCG) if available
  • Previous relevant history
  • Appropriate section 88 code
  • Previous caesarean section.

Early pregnancy ultrasound examination

  • First trimester dating should be determined by crown-rump length (CRL) not from mean gestational sac diameter as the latter is less accurate.
  • Estimated date of delivery (EDD) and gestational age (GA) from known in vitro fertilisation (IVF) dates should not be changed.
  • Most early pregnancy scans less than 7 weeks will require transabdominal (TA) and transvaginal (TV) imaging.
  • TA imaging alone may be sufficient if excellent visualisation of a live intrauterine embryo is achieved and the woman is asymptomatic.
  • TV imaging should always be offered when the indication includes abnormal symptoms, such as bleeding or pain.

Early pregnancy scans should include the following as a minimum.

  • Determination of the gestational sac location
  • Imaging of the relationship of the sac to previous caesarean section scar (see Appendix 2: Low gestational sac in the first trimester with previous caesarean section)
  • Imaging of yolk sac
  • Imaging of embryo (fetal pole)
  • CRL
  • Imaging of embryonic cardiac activity – document with a cine clip if possible; otherwise annotate cardiac activity present or absent on the relevant image
  • Documenting of dates by CRL (see below)
  • An assessment of region(s) of haemorrhage
  • An assessment of the chorionicity and amnionicity of multiple pregnancy (PDF, 318 KB) (see NZMFMN 2015f)
  • An assessment of myometrium, adnexa and degree of free fluid in the pelvis.

See also the sections to follow on:

  • Normal early intrauterine pregnancy
  • Early pregnancy loss
  • Ectopic pregnancy and pregnancy of unknown location.

Reporting guide and referral recommendations

Minimum reporting requirements

Scan technique (TA/TV). Document if the woman declined a TV scan.

Document findings as per Early pregnancy ultrasound examination (see above):

  • sac location (see Appendix 3: Ectopic pregnancy location)
  • CRL (date pregnancy if embryo is present)
  • mean sac diameter (MSD) if no embryo identified
  • cardiac activity
  • EDD by CRL if embryo is present
  • uterine and adnexal masses (such as fibroids, ovarian cysts, dermoids and other findings
  • free fluid
  • significant haemorrhage (small asymptomatic haemorrhages do not need to be reported).

Guide to assessing gestational age with ultrasound

The following guidelines are adapted from the 2017 ASUM First Trimester Ultrasound Standard of practice.

The CRL provides the most accurate estimation of gestational age in the first trimester.

Before an embryo is visible, the MSD can support gestational age by LMP but should not be used to determine EDD.

Once a live embryo is visible, the CRL should be used to calculate the due date. The MSD should not be included in this calculation.

After 11 weeks, multiparametric assessment can be used with biparietal diameter (BPD) being the most often used second measurement.

Accuracy of dating:

  • ± 4 days by CRL at ≤10 weeks
  • ± 7 days by biometry from 10–20 weeks
  • ± 14 days from 20–30 weeks
  • ± 21 days from 30 weeks.

EDD by assisted reproduction dates (eg, IVF) should not be adjusted.

In the presence of twins, the CRL for the larger twin is used in assessing the EDD.

For more information, see the Guidelines for the Performance of First Trimester Ultrasound G02 (PDF, 233 KB) (ASUM 2017).

For reporting pro forma examples, see First trimester reporting pro forma.

Reporting alerts

  • Ruptured ectopic pregnancy
  • Ectopic without evidence of rupture
  • Early pregnancy loss
  • Suspected molar pregnancy
  • Implantation of the sac on the caesarean scar, see Appendix 2 (this indicates a risk of placenta accreta spectrum disorder and requires specialist referral).
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