Anatomic Survey of the Fetus and It’s Relationship to the Gestational Age

Joserizal SerudjiHKFM Padang

Ultrasound milestones

• There is a major difference between when a certain finding can appear and when it should be present( the latter having more importance in early pregnancy failure).

• One can identify structures much earlier with transvaginal ultrasound (TVU), which has a separate timetable (up until the eleventh week there is little reason to view a first trimester pregnancy with TAU other than as an initial quick scouting.

• The common tomograms (slices) used to assess a portion of the fetal anatomy are the axial slice, the coronal slice, and the sagittal slice, most often midline.

Schematic of the essential planes for ultrasound scanning: axial (a) and coronal planes (b), sagittal (midline) (c) incorporating. T, thalami; C, cavum; CC, corpus callosum; CV, cerebellar vermis. For example, (a) is plane of the biparietal diameter.

Several ultrasound parameters have been used to estimate gestational age. The most commonly used are:

• ● mean sac diameter• ● gestation sac volume• ● crown–rump length (CRL)• ● biparietal diameter (BPD)• ● femur length

The different ultrasound parameters used to estimate gestational age

Gestational age (weeks) Parameter4+ to 6+* Mean sac diameter4+ to 6+* Gestation sac volume6+ to 12* Crown–rump length12 to 15 Defer measurement15 to 24 Biparietal diameter, femur length and circumference measurements24 weeks onward Gestational age cannot be

accurately determined by ultrasound

*Recognition of a gestation sac from 4+ weeks and a fetal pole from 6+ weeks will normally be achieved only with a transvaginal probe

Gestational sac

• The first ultrasound sign of pregnancy• Is generally oblong and has a thick “rind” • Should have a double ring, representing the decidua

capsularis and the decidua parietalis• Should be seen when the beta human chorionic gonadotropin

(hCG) is between 1000 and 2000 mIU/mL.• Once seen, the sac diameter should grow by an average of 1

mm a day• The mean sac diameter (MSD) can be used as a gauge against

which to assess other findings• Beware of the pseudosac, which does not have a double ring

and is seen in association with ectopic pregnancy

Early gestational sac

Sac diameters

Yolk sac

• The second structure to be visible by ultrasound • Can be seen when the MSD is 5 mm, but it should be seen by

the time the MSD is 8 mm• Plays a crucial role in the development of the fetus—providing

nourishment and producing the stem cells that develop into red blood cells, white blood cells, and platelets

• Provides the immunological potential for the fetus until about 7 menstrual weeks, when those functions are taken over by the fetal liver

• From then on the functionless yolk sac becomes a circular structure without a core, after which it finally disappears by 12 menstrual weeks.

Yolk sac

Embryo

• Could be seen by 5 menstrual weeks and a way to determine gestational age is to add 42 days to the crown–rump length (CRL) measurement in millimeters.

• Should increase its CRL by 1mm/d.• Not seeing an embryo when the MSD has reached 6 mm is

indicative of apregnancy loss• The size of the embryo, relative to the MSD, is important if

the MSD–CRL is <6 mm, the prognosis is very poor.• Cardiac activity should be visualized when the embryonic

length is greater than or equal to 4 mm not seeing a beating heart at this embryonic size is an ominoussign

Normal first trimester fetus with frontal echo spared area

Should see on TVS Time of visualization

Gestation sac 5 menstrual weeksYolk sac when MSD is >7mmEmbryonic pole 5 weeks or when hCG is>1000 mIU Fetal heart activity when CRL is >5 mm

• In the majority of normal pregnancies, measurement of the biparietal diameter (BPD) and femur length (FL) provide the most accurate assessment of gestational age in the second trimester.

• It is recommend that measurements of the head circumference (HC), transcerebellar diameter (TCD) and abdominal circumference (AC) are also undertaken.– They provide further confirmation of gestational age and

aid in the exclusion of growth related abnormalities and spina bifida.

– In addition, their inclusion encourages a systematic examination of the whole fetus

Biparietal diameter (BPD)

• This was the first fetal measurement attempted, first by A-mode and then later by 2D contact scanning.

• The measurement is made axially at a level just above the ear and, although others have obsessed over standardization, it is very difficult to mess up this measurement.

• BPD is measured at the level of the thalami and is made from the outer margin of the calvarium to the inner surface of the downside of the skull.

• The interobserver variability is small, and one can rely on this measurement as one of the best indicators of gestational age.

Plane of the biparietal diameter

• A single optimal measurement of the BPD will predict the gestational age to within ± 5 days. It is more accurate at predicting the date of delivery than an optimal menstrual history. This last point has justified its use in all pregnancies

• There are multiple formulas correlating a given BPD with gestational age, but most clinicians and sonographers simply use the formula that is already in the software of their ultrasound machines.

• There are only modest differences in BPDs across different populations, so using a built-in formula is acceptable.

Biparietal diameter (BPD) (cont’ed)

Head circumference (HC)

• This measurement puts into play a diameter made east to west across the skull, which depends upon the inferior lateral resolution of the transducer, and is also dependent upon the degree of fetal tucking.

• Perhaps its greatest benefit is when, -because of either oligohydramnios or a breech presentation, -there is dolichocephaly, rendering the BPD less accurate.

• Most clinicians will use both measurements to evaluate fetal head size.

Measurement of head circumference using the ellipse method (lateral ventricles

view)

Transcerebellar diameter (TCD)

• The best dater of pregnancy, and it is not difficult to obtain, at least in the second trimester.

• The TCD in millimeters roughly equals gestational age in weeks until about 22weeks, after which a nomogram is needed

• It is therefore useful in the assessment of gestational age, especially when there is a discrepancy in gestational age equivalent between the BPD or HC and the femur

• It is found that TCD to be very useful in late care patients to sort out an SGA fetus from an off-on-dates fetus.

Measurement of the transcerebellar diameter. The calipers are positioned to obtain an ‘outer to outer’ measurement of the cerebellum. The cerebellar vermis (CV), cerebellar hemispheres (CB), cisterna magna (CM), nuchal skin fold (NF)

• The bonus with the TCD is that it forces the operator to image the entire posterior fossa, which is a rich source of information regarding the cerebellar vermis itself and, indirectly, is a reflector of the integrity of the neural tube.

• In addition, in the same plane the nuchal skin fold thickness, -our best marker of Down syndrome-, can be measured

Transcerebellar diameter (TCD) (cont’ed)

Abdominal circumference (AC)

• The best one to assess the size of the fetus and, indirectly, the status of fetal nutrition

• The worst way, by itself, to determine gestational age, because of its wide biological variation

• AC was originally chosen as a standard part of the biometric profile is because the measurement is made at the level of the bifurcation of the right and left portal veins, e.g., smack in the middle of the liver, an organ that is quite large in macrosomia and very small in small-for-gestational age (SGA) fetuses

• The measurement must be precisely performed because it is, unfortunately, easy to get a tangential cut through the abdomen, overestimation of the abdomen size; especially when a large portion of the umbilical vein is incorporated in the image

The abdominal circumference

Femur and humerus

• The femur is indirectly reflective of the crown heel length of the fetus (length in cm times 6.9) and is subject to familial tendencies toward shortness or tallness.

• The humerus simply adds more to the biometric mix, but is not included in formulas to estimate fetal weight, as is the femur

• The measurement of the femur and humerus should only include the ossified shaft

The femur length is the distance between the caliper markers

Estimating gestational age

• The common practice is to take an average of the biometry (BPD, HC, AC, femur) as an average ultrasound age (AUA) then be compared with the patient’s menstrual dates.

• The dating precision varies according to the gestational age of the patient.

• It used to be thought that the CRL was the most accurate way to date pregnancy, but the BPD in the second trimester has very reasonable accuracy.

• Although TCD is a very good indicator of gestational age in the second trimester, it is, by far, the best dater of pregnancy in the third trimester, because it rarely is affected appreciably by aberrations in fetal growth

Discordant measurements

BPD and HC• For practical purposes, a discrepancy between the fetal head

measurements and the patient’s dates of greater than 2weeks should get the attention of the ultrasound examiner.

• If the head is bigger, then a quick evaluation of the intracranial anatomy should assure the examiner that this relative macrocephaly is simply a normal variant and,most often, a familial feature

• If the head circumference is smaller by about 2weeks, this most often is a normal variation, especially in the third trimester. However, the larger the discrepancy, the more one will have to explore the rare condition of pathologic microcephaly

Discordant measurements (cont’ed)

AC• By far the most common reason for a small AC is IUGR, and if

it is out of sync with the rest of the biometry, it represents early asymmetric growth restriction. Gastroschis is associated with a small AC, but, fortunately, in the scheme of things, this is an uncommon condition and is easily diagnosed.

Discordant measurements (cont’ed)

Long bones• Long limbs generally signify tall genes; and there are only a

few rare circumstances, such as in Marfan syndrome• Almost every day on a busy ultrasound service a fetus shows

up with a femur that is smaller than expected.• When the discordance is in the 2- to 3-week range, this

virtually always represents a genetic predisposition toward shortness; especially when found well into the third trimester,

• Unfortunately, since a short femur may be a marker for Down syndrome, other markers, including the humerus (which generally is even smaller in Down syndrome than the femur) need to be assessed

Discordant measurements (cont’ed)

Long bones (cont’ed)• If the fetus is very short (greater than a 3-week discrepancy),

then two other clinical avenues need to be explored. Very early primary placental growth failure will sometimes emerge with a short femur representing the first biometric measurement to fall off the growth curve. Most often this occurs together with abnormal uterine artery waveforms

• A very short femur should trigger a short-limb dysplasia investigation, which, as indicated later, is quite extensive.

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