Depending on the age of the gestation, these graphs can be used to determine the correct EDD.
Mean Sac Diameter (MSD) 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
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.
‘If the mother has a retroverted uterus or fibroids, the GS may be difficult to identify.
The GS is an echogenic ring surrounding an anechoic centre.
An ectopic pregnancy will appear the same but it will not be within the endometrial cavity.
The GS is not identifiable until approximately 4 1/2 weeks with a transvaginal scan (sometimes not visible until even later, depending on the patient)
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 often be seen during week 4-5.
Ultrasound image 5 week gestation. Yolk Sac Only seen.The yolk sac will be visible before a clearly definable embryonic pole.
Ultrasound image- 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 .
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 a maternal 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 on ultrasound after the GS.
It should be round with an anechoic centre.
It should not be calcified, misshapen or >6mm from the inner to inner diameter.
Yolk sacs larger than 6 mm are usually indicative of an abnormal pregnancy.
Failure to identify a yolk sac(with transvaginal ultrasound) when the gestational sac has grown to 12 mm is also usually indicative of a failed pregnancy.
A misshapen yolk sac is a higher predictor of a poor outcome than size of the yolk sac.
The yolk sac should be circular with a hyperechoic rim and anechoic centre.
Trans vaginal ultrasound at 5 weeks gestation: yolk sac and fetal pole visualised.
Using a transvaginal approach the fetal heart beat nay be seen flickering before the fetal pole is even identified or measurable.
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 sonographers will take the mothers pulse or doppler a maternal artery at the same time to check if it is the fetus or the mothers .
Cardiac activity is visualised using M mode during a transvaginal scan.
Crown Rump Length (CRL)
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 supplemented 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 neuropore, 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
Ultrasound image- 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.
Ultrasound image- This is a foetus at 10 weeks gestation.
There is significant change with features of the foetus more discernable.
Ultrasound image- This is a normal sonographic appearance of a midgut herniation. There is out-pouching of the anterior abdominal wall with a small portion of bowel encapsulated by a membrane within the base of the umbilicus. It usually measures between 4 and 7 mm.
10 week heart rate
Ultrasound image- 10 week legs
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.
Ultrasound image- The fetal face has begun to take shape. Look for symmetry.
Ultrasound image- 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.
Ultrasound image- At 10 weeks, visualise 4 jointed limbs,feet and hands.
The legs are usually crossed at the ankles. Confirm the presence and symmetry of the long bones in Ultrasound image.
Ultrasound image showing the correct angle of 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.
Ultrasound image- The humerus, radius and ulna and the presence of hands are imaged from 11 weeks
Ultrasound image 12 week choroids take up most of the space within the ventricles.
Role of Ultrasound
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
Exclude an ectopic pregnancy
Maternal past history
Nuchal Translucency (11-14 weeks : CRL 45-84mm)
Parity (Miscarriage, Termination of
Date of Last Menstrual Period
Other pregnancy History
If a transvaginal approach is not done when the gestation is less than 8 weeks dating can be inaccurate, particularly in a retroverted uterus.
If there are multiple fibroids visualisation is difficult.
Maternal body habitus is always a factor in pregnancy ultrasound, particularly a BMI >30
90 minutes before the appointment time, empty your bladder.
Over the next hour, drink at least 750mls of water and do not go to the toilet until instructed.
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.
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. For example with twins: 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
A 1st trimester series should include the following minimum images;
Uterus – long, trans
Cervix and Pouch-Of-Douglas
Gestational sac – Long & Trans
Yolk sac if visible
M mode fetal heart
Document the normal anatomy. Any pathology found in 2 planes, including measurements.