12-16week anatomy

Role of Ultrasound

This scan reviews the fetal viability, number, structure and/or to assess the likelihood of aneuploidy. Ideally performed 12 +4 weeks or greater, unless there are clinical concerns

Common indications:
1. Early structural scan
2. Nuchal translucency screen
3. Pre-eclampsia screen
4. Per-vaginal bleeding
5. Pelvic pain

Clinical History 

a. Reason for scan
b. LMP, EDC and or embryo transfer date
c. Relevant clinical, surgical or family history including previous urine or serum HCG levels

Crown Rump Length (CRL)

The CRL is used to assess the fetus size and determine the gestational age up to 14 weeks.. 

The CRL is measured between the fetal poles, excluding the limbs.

The ultrasound image should be magnified to take up most of the image horizontally, with the fetus measured in a mid sagittal position. The neck is in a neutral position with fluid between the chin and chest.


Ultrasound image- The falx is seen in the midline of the brain with the choroids taking up most of the ventricular space.

Ultrasound image- This is a normal axial view of the brain which shows the normal shape of the calcified skull and the sutures.

Ultrasound image- In this axial plane of the fetal midbrain the following structures are visualised

Red – Aqueduct of Sylvius (AOS)

Yellow – Thalamus

Blue – 3rd ventricle

A measurement of AOS to occiput distance should be measured from the posterior border of the AOS to the anterior border of the occiput bone as a guide for a marker of spina bifida.

Ultrasound image- In this sagittal view the following posterior fossa structures can be visualised

Orange- Thalamus/Brainstem

Blue- Intracranial Translucency (ICT)

Brown- Cisterna Magna

Yellow- Nuchal Translucency

The Nuchal Translucency is used to provide a risk assessment for chromosomal abnormalities, specifically Trisomies 13,18 and 21 (Down’s Syndrome). This is a risk assessment based on age, heritage,history and a specific ultrasound measurement at the back of the fetus neck. The accuracy of this is increased by factoring in the levels of bHCG and PappA in the maternal blood. For more details go to the following link: The Fetal Medicine Foundation.

Facial Structures

Ultrasound image-  In this coronal view the orbits and lenses are visualised.

The retronasal space (retronasal triangle) is posterior to the nose which is visualised by scanning posteriorly in the coronal plane.

The three echogenic lines show the two frontal processes of the maxilla and the palate.

Ultrasound image-

The profile view shows :

1. Nasal Bone (yellow)

2. Maxilla (blue)

3. Mandible (green)

The tip of the nose , and the angles of the mandible and maxilla are important to visualise to rule out facial dysmorphism and anomalies such as micrognathia associated with chromosomal abnormalities.

The profile should be imaged in a mid sagittal plane with the angle of the face at approximately 45degrees. The level of calcification and the full length of the nasal bone should be noted. An absent or hypoplastic Nasal Bone is associated with Down Syndrome (Trisomy 21). There should be no frontal bossing (sloping) of the forehead. The tip of the mandible, upper and lower lips and tip of the nose should be in line or micrognathia should be suspected.


Ultrasound image- Assessment of a correctly positioned 4 chamber heart and closed/open  AV valves are seen in both B Mode and colour. It is important to watch the valves both open and close completely, particularly tricuspid regurgitation which may be associated with chromosomal and structural abnormalities.

Ultrasound image- The Right ventricular outflow tract (RVOT) or main pulmonary artery is visualised in both B mode and colour. The blood flow is moving out of the rt ventricle towards the spine. The asscending aorta is seen to the right of the RVOT. This outflow tract should bifurcate and move behind the aorta (seen in colour).

Ultrasound image- The left ventricular outflow tract is seen coursing across the heart from the left to the right side. It should not be parallel to the RVOT.

Ultrasound image- The “Arrowhead view” displays the transverse portion of the aorta merging with the pulmonary artery to the right of the trachea. Ensure there is a “V” shape and not a “U” shape as this may indicate a vascular ring.

Abdominal Structures

This ultrasound image shows both the stomach and the heart position within the fetus with the left side labelled to ensure there is correct positioning of the stomach and heart.

This colour doppler ultrasound image shows the cord insertion with the two umbilical arteries. It is important to ensure there are no bowel contents protruding into the cord or floating free within the amniotic fluid.

Ultrasound image- The left and right diaphragms should be assessed for any herniation of abdominal contents in the thorax.

Ultrasound image- The bladder should always be visualised and if possible both the kidneys.

Ductus Venosus (DV)

The DV is best sampled using pulsed wave Doppler with a gate of 1-2 mm. It is placed at the isthmus, near the DV origin from the umbilical vein to obtain a waveform with the smallest possible angle of insonation.

The waveform is assessed by observing if there is any reversed or absence of an a-wave.


Ultrasound image-Sagittal spine with the skin line visualised with the baby in a prone position.

Ultrasound image- A coronal view of the spine to rule out skeletal anomalies such as scoliosis.


Ultrasound image- It is important to image the long bones in the upper limb.  5 fingers should be observed opening and closing.

Ultrasound image- Both the femora, tibia and fibulae should be identified. In particular the shape and boney mineralisation to rule out any congenital skeletal defects such as bowing.

Ultrasound image- Both feet should be identified at a  90 degree angle  from the lower leg, to rule out talipes. It is very difficult to see 10 toes confidently.


Patient History

  • Gravidity
  • Parity (Miscarriage, Termination of
  • Pregnancy (T.O.P))
  • Fertility treatment
  • Date of Last Menstrual Period
  • Other pregnancy History
  • Gynaecological History


  • If there are multiple fibroids visualisation is difficult.
  • Maternal body habitus is always a factor in pregnancy ultrasound, particularly a BMI >30
  • retroverted uterus (can be overcome with a transvaginal scan
  • Contractions

Patient Preparation

  • 2 hours before the appointment time, empty your bladder.
  • Over the next hour, drink at least 1 litre of water and do not go to the toilet until instructed.

Equipment setup

  • Modern ultrasound unit
  • Curved linear probe approx 3-7 MHz depending upon maternal factors
  • Transvaginal probe approx 5-9 MHz (Use of non-latex cover if required)
  • Ensure patient comfort and privacy.
  • Warm gel, clean towels etc
  • Select “Obstetric” preset for appropriate power levels and measurement packages
  • Use a curvilinear probe (3.5-6MHZ) with low power to reduce risk of biological effects.
  • Use of doppler should be avoided in the 1st trimester.

Common Pathology

  • Extramembranous haemorrhage
  • Fetal demise
  • Blighted ovum/Anembryonic pregnancy
  • Structural abnormalities


  • Cervix – assess if closed and measure length between internal and external os
  • Look for bright trophoblastic reaction around sac.
  • Assess placental location and distance from internal os (may lie close to os at this stage)
  • Check for retroplacental haemorrhages, placental masses etc
  • Assess maternal ovaries, adnexae and
  • Pouch Of Douglas (P.O.D)
  • Confirm presence of intrauterine gestation, and number
  • If multiple pregnancy, confirm number of foetuses, number of sacs, and number of placentas present to determine chorionicity.  ie Monochorionic/Monoamnionic(MCMA), Monochorionic/Diamnionic(MCDA), Dichorionic/Diamnionic(DCDA)
  • Confirm heart beat(s) & rate with M-Mode only (Use of Colour or Doppler traces is not recommended in the 1st trimester)
  • Measure CRL to calculate gestational age and Estimated Date of Delivery(EDD).
    If too early to see the foetal pole measure the average sac diameter.

Basic Hardcopy Imaging

Transabdominal imaging:
a. Longitudinal and transverse uterine views
b. Fetal heart motion, if possible
c. Transverse adnexa
d. Ovaries, if visualised
e. Maternal Kidneys, if clinically relevant

Transvaginal or transabdominal images:
a. Cervix: Longitudinal
b. Uterus:
i. Longitudinal and transverse
ii. Position of gestation sac
iii. Presence of extra-membranous haemorrhage (measured in 3 planes and volume in mL)
c. Gestation sac and pregnancy:
i. Gestation sac: measured in 3 planes and mean sac diameter calculated if no fetal pole present or unless clinically indicated
ii. Yolk sac: presence and size
iii. Fetal pole. Identify number and measure crown rump length (with fetus in neutral position)
iv. Chorionicity, amnionicity and fetal position (superior/inferior, left/right) in the presence of twins and higher order multiple. A drawing on the report template may be of value in higher order multiples and complex cases.
v. Fetal structures imaged
-Fetal Heart Rate 
-CRL (mm)
-BPD (mm)  , HC (mm)  , FL (mm) – when CRL is greater than 84mm 
-NT (mm) – NF if CRL is greater than 84mm
-Nasal Bone
-Retro nasal angle
-Falx/ Choroid
-Thorax/Diaphragm (lt and rt) 
-Fetal Cord Insertion
-Fetal Bladder
-Umbilical Arteries
-Both Femurs
-Both Tibs and Fibs
-Both Feet – plantar view where possible
-Both Humeri
-Both Radius / Ulna
-Both Hands
-Fingers (show each digit)
-4CH Heart
-4CH Heart – CDI
-Outflow tracts – CDI
-Intracranial translucency
-Spine – skin line
-Tricuspid Valve PW doppler Trace
-Ductus Venosus PW Doppler Trace
-If possible the orbits, lenses, kidneys
vi. When performing a Nuchal Translucency scan, 3 x Nuchal Translucency Measurements that meet Nuchal Accreditation Standards. If there is significant discordance, further images should be taken. Nuchal cord should be documented.
d. Ovaries:
i. longitudinal and transverse
ii. 2D and colour Doppler
iii. Identify corpus luteum
e. Adnexa
i. At least one image each adnexa
f. Pouch of Douglas image

Additional images will be required in the presence of pathology, for example, fibroids, Mullerian anomalies and ovarian masses.