For the scanning protocol




Depending on the age of the gestation, these graphs can be used to determine the correct EDD.

Mean Sac Diameter measurement is used to determine gestational age before a Crown Rump length can be clearly measured.The average sac diameter is determined by measuring the length,width and height then dividing by 3 .
 Once a fetal Pole can be visualised the CRL measurement is the most accurate method for dating the pregnancy


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HCG Levels in Normal pregnancy
HCG Levels for normal Pregnancy.NOTE: The quantitative maternal serum beta HCG peaks at approximately 10 weeks and then reduces.



The gestational sac(GS) is the earliest sonographic finding in pregnancy. It will be difficult to see if the mother has a retroverted uterus or fibroids. The GS is an echogenic ring surrounding an anechoic centre. An ectopic pregnancy will appear the smae but it will not be within the endometrial cavity. The GS is not identifiable until approximately 4 1/2 weeks with a transvaginal scan.

Gestational sac size should be determined by measuring the mean of three diameters. These differences rarely effect gestational age dating by more than a day or two.


The following image is using a transvaginal approach the gestational sac can be seen during week 4-5.




5 week gestation. Yolk Sac Only seen.The yolk sac will be visible before a clearly definable embryonic pole.  Mean Sac Diameter measurement is used to determine gestational age before a Crown Rump length can be clearly measured.The average sac diameter is determined by measuring the length,width and height then dividing by 3 .
Fetal Heart Rate 6 week gestation
The very early embryonic heart will be a subtle flicker.This may be measured using M-Mode(avoid Doppler in the first trimester due to risks of bioeffects).Initially the heart rate may be slow.Compare to the maternal heart rate to confirm that you are not seeing an arteriole.
The Crown Rump Length (CRL) measurement in a 6 week gestation.A mass of fetal cells, separate from the yolk sac, first becomes apparent on transvaginal ultrasound just after the 6th week of gestation. This mass of cells is known as the fetal pole.  

The fetal pole grows at a rate of about 1 mm a day, starting at the 6th week of gestational age. Thus, a simple way to "date" an early pregnancy is to add the length of the fetus (in mm) to 6 weeks. Using this method, a fetal pole measuring 5 mm would have a gestational age of 6 weeks and 5 days.




The yolk sac appears during the 5th week. It is the second structure to appear after the GS. It should be round with an anechoic centre. It should not be calcified, misshapen or >5mm from the inner to inner diameter. Yolk sacs larger than 6 mm are usually indicative of an abnormal pregnancy. Failure to identify (with transvaginal ultrasound) a yolk sac when the gestational sac has grown to 12 mm is also usually indicative of a failed pregnancy.



Using a transvaginal approach the fetal heart beat can be seen flickering before the fetal pole is even identified. It will be seen alongside the yolk sac. It may be below 100 beats per minute but this will increase to between 120- 180 beats per minute by 7 weeks. In the early scans at 5-6 weeks just visualising a heart beating is the important thing. Failure to identify fetal cardiac activity in a fetus whose overall length is greater than 4 mm is an ominous sign.

Sometimes there is difficulty distinguishing between the maternal pulse and fetal heart beat. Often technicians will take the mothers pulse at the same time to check if it is the fetus or the mothers .


The CRL is a reproducible and accurate method for measuring and dating a fetus.

Early ultrasonographers used this term (CRL) because early fetuses also adopted the sitting in the chair posture in early pregnancy.   After 12 weeks, the accuracy of CRL in predicting gestational age diminishes and is replaced by measurement of the fetal biparietal diameter. 

In at least some respects, the term "crown rump length" is misleading:

  • There is no fetal crown and no fetal rump to measure for most of the first trimester. 

  • Until 53 days from the LMP, the most caudad portion of the fetal cell mass is the caudal neurospone, followed by the tail. Only after 53 days is the fetal rump the most caudal portion of the fetus.

  • Until 60 days from the LMP, the most cephalad portion of the fetal cell mass is initially the rostral neurospore, and later the cervical flexure. After 60 days, the fetal head becomes the most cephalad portion of the fetal cell mass.

  • What is really measured during this early development of the fetus is the longest fetal diameter.

From 6 weeks to 9 1/2 weeks gestational age, the fetal CRL grows at a rate of about 1 mm per day.



8 WEEKS                                                                                      10  WEEKS

8 weeks 10 weeks
A normal 8 week foetal pole. You should see a definable head and body. The beginning of the limb buds. The fetal heart should be easily visible. Subtle body movements can often be seen.
10 weeks
10 week heart rate 10 week legs

10 weeks Fetal Face

The 2 sacs are clearly visible.

The outer chorion with the developing placenta and the inner amnion which will "inflate" with the production of fetal urine,to adhere to the chorion obliterating the residual yolk sac.

The normal small mid-gut hernia into the cord is still visible.(pink shading). This is the result of normal midgut proliferation and will resolve by 11 weeks as the fetus lengthens. This physiological occurrence should not be confused with an omphalocele.

The fetal face has begun to take shape. Look for symmetry.

10 week CRL joints
Measure the crown rump length (CRL) to estimate gestational age. The rhombencephalon of the developing brain is visible as a prominent fluid space posteriorly. This should not be mistaken for neck oedema or other pathology.
At 10 weeks, visualise 4 jointed limbs,feet and hands.

12 weeks NT
From 12 weeks the basic morphology of the fetus is visible
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.
12 week legs angle of feet
The legs are usually crossed at the ankles. Confirm the presence and symmetry of the long bones
The correct angle the feet to legs can be confirmed. They should be at 90 degrees ie perpendicular or Talipes should be suspected. This can be confirmed over the following weeks.

arms and hands 12 week choroids
The humerus, radius and ulna and the presence of hands are imaged from 11 weeks.
12 week choroids take up most of the space within the ventricles.


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Initially twins may be identified as 2 separate gestational sacs (ie diamniotic, dichorionic) They may be 2 fetal poles within the same gestational sac (monochorionic). It is easier to determine chorionicity earlier in the pregnancy depending on the chorionicity and amnionicity.

It is a sad situation when a "vanishing twin" occurs, which is about 20% of twin pregnancies. In these cases, one of the twins fails to grow and thrive. Instead, its development arrests and it is reabsorbed, with no evidence at delivery of the twin pregnancy.


 Monochorionic Twins  Dichorionic Diamniotic Twins
Monoamniotic Twins
Dichorionic diamniotic Twins




Triplets with 2 sacs.Monoamniotic,monochorionic twins and a normal single.
Click here for a great article on twins







For First Trimester ABNORMALITIES


  • miscarriage
  • Ectopic Pregnancy
  • Conjoined Twins
  • Antepartum Haemorrhage


For uterine artery assessment





Ultrasound is essentially used for assessing gestational age, current viability and maternal wellbeing. Ultrasound is a valuable diagnostic tool in assessing the following indications;

  • Unsure of Dates
  • Vaginal Bleeding
  • Pelvic Pain
  • Exclude an ectopic pregnancy
  • Maternal past history
  • Threatened Miscarriage
  • Nuchal Translucency (11-14 weeks : CRL 45-84mm)

Patient History

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


  • 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.




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.

<10 weeks
  • 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.



A 1st trimester series should include the following minimum images;

  • Uterus - long, trans
  • Both ovaries
  • Adnexae
  • Cervix and Pouch-Of-Douglas
  • Gestational sac - Long & Trans
  • Yolk sac if visible
  • Fetal pole
  • M mode fetal heart
  • Document the normal anatomy. Any pathology found in 2 planes, including measurements.

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