Imaging In Obstetrics | First, Mid And Third trimester Ultrasound

Ultrasound in obstetrics
 First Evidence Of Pregnancy
First Trimester ultrasound
Second Trimester Ultrasound
Third Trimester Ultrasound
Yolk Sac

Imaging in obstetrics

Ultrasound in obstetrics

Diagnostic ultrasound has been used in obstetrics for nearly 30 years. Although generally considered safe, there is continuing study and research to confirm this. It is a very important technique for examining pregnant women and can be used when clinically indicated at any time during pregnancy.

Which stage of pregnancy provides better information in ultrasound scans?

There are two stages during a normal pregnancy when ultrasound will be the most useful and provide the most information.

These stages are:
  1. At 18-22 weeks after the first day of the woman's last menstrual period
  2. At 32-36 weeks after the first day of the woman's last menstrual period

When is ultrasound not recommended?

There is no indication for an ultrasound examination in the first trimester of pregnancy unless there is a clinical abnormality.

Why is a scan not recommended at the mother's first visit?

Some physicians do recommend an ultrasound examination at the time of the mother's first visit, but there is no reason to do this provided the clinical examination is normal. When considered necessary, scanning during weeks 18-22 of pregnancy will provide much more important information.

There is no need to perform an ultrasound every month or during every antenatal visit unless there is a clinical reason to suspect an abnormality that needs to be investigated further.

Why consider scanning during a normal pregnancy?

Many physicians that scanning is unnecessary during a clinical normal pregnancy. Others recommend scanning because many obstetric abnormalities cannot be detected by clinical examination.
  1. 90% of developmental fetal abnormalities occur without any family history and very few of the mothers show any obvious risk factors.
  2. There can be significant fetal abnormalities even in a clinically normal pregnancy.
  3. Neither clinical examination nor family history is an entirely reliable way to detect multiple pregnancies.
  4. A significant number of mothers with a low-lying placenta (placenta praevla) show no evidence until bleeding starts at the onset of labor. The situation can then be extremely dangerous, especially if the patient is a long way from the nearest hospital.
  5. Up to 50% of mothers who claim to know their obstetric dates with certainty are in fact more than two weeks in error when gestational age is calculated with ultrasound. A discrepancy of two weeks can be critical for an infant's survival who has to be delivered early because of some antenatal complication.

What are the objections to scanning during a normal pregnancy?

Many physicians believe that the possible risks and the cost of scanning every clinically normal pregnancy are not justified by the benefits for the patients.

To scan or not to scan a normal pregnancy, this decision must be made by the physician and each patient. There are no universally accepted guidelines at present.

The determination of the sex of the fetus is not a valid indication for ultrasound except when there is a strong familial risk of a sex-linked genetic disorder.

The first evidence of Pregnancy (obstetrics)

The first evidence of pregnancy is the location of the gestational sac. It can often be recognized in the uterus after five weeks of amenorrhea and may be located asymmetrically.

All normal pregnancies should be recognizable after 6 weeks as a well-defined "double echogenic ring" in the uterus. The inner ring is of uniform echogenicity and is 2mm or thicker. Around it is a thin echogenic ring, which does not encircle the entire gestational sac. Between the two rings is the anechogenic residual uterine cavity.

At 5-6 weeks, the greatest diameter of the gestational sac is approximately 1-2 cm. At 8 weeks the sac should occupy half the uterus; at 9 weeks it should take up two-thirds of the uterus, and at 10 weeks it should fill the uterus.

The gestational age can be estimated to within one week from the mean dimension of the sac. Using a longitudinal scan, measure the maximum dimensions of the sac in the long axis (length), and at 90d to this in the anteroposterior (AP) dimension. Make a transverse scan at right angles to the longitudinal scan plane and measure the greatest width of the sac. The mean dimension of the sac is the sum of these three measurements divided by 3.

       Mean gestational sac dimension = Length + AP + Width
The gestational age of the fetus can be estimated by reference to local standard development tables.

What could be seen in First-trimester ultrasound

An intrauterine gestational sac (GS) should be seen by transvaginal ultrasound (TVS) when the maternal serum β-hCG level is 1,000-1,200 mIU/mL and by transabdominal ultrasound with the level of β-hCG 6,000 mIU/mL. The yolk sac is seen at the level of 7,000 mIU/mL and the embryo at 11,000 mIU/mL. Definite diagnosis of intrauterine pregnancy is possible as early as 29-35 days of menstrual age, In the first-trimester ultrasound. 

The true gestational sac (GS) is eccentric in position within the endometrium of the fundus or body of the uterus. The double decidua sign of the gestational sac is due to the interface between the decidua and the chorion which appears as two distinct layers of the wall of the gestational sac.

What confirms pregnancy?

The presence of a yolk sac or fetal pole within the gestation sac confirms pregnancyTrue gestational sac size increases 1 mm/day. Pseudogestational sac or pseudo sac is irregular in outline, usually centrally located in the uterus, has no double decidua sign and the sac remains empty. The rate of early (<12 weeks) pregnancy loss (miscarriage) diminishes steeply with the progressive appearance of fetal structures (e.g. with only GS=11.5% and with embryo >10mm=0-5%).

How to estimate Gestational age dating in pregnancy

Ultrasound examination is the best method to estimate gestational age dating. The error with LMP is due to ovulation (>14 days after LMP).

CRL is most accurate with an error of 2.1 days in the first trimester. Biparietal diameter (BPD), femur length (FL), head circumference (HC), and abdominal circumference (AC) are commonly used for dating thereafter.

5 major ultrasound markers for fetal anomalies during first-trimester ultrasound

1. Nuchal translucency: Increased fetal nuchal skin thickness (in the first trimester) >3 mm by TVS is a strong marker for chromosomal anomalies (trisomy 21, 18, 13, triploidy Turner's syndrome).

2. Multiple pregnancies: Identification of two gestational sacs indicates twin birth in 52-63% of cases.

3. Anembryonic pregnancy (blighted ovum): It is a sonographic diagnosis. There is the absence of a fetal pole in a gestational sac with a diameter of 25mm or more uterus is to be evacuated once the diagnosis made.

4. Ectopic pregnancy: TVS can detect 90% of tubal ectopic pregnancy. The double decidual sac sign differentiates normal pregnancy from the pseudogestational sac of an ectopic pregnancy.

5. Hydatidiform mole: It is an abnormal condition of the placenta where there are partly degenerative and partly proliferative changes in the young chorionic villi.

Indications of the first-trimester ultrasonography

◾ Intrauterine pregnancy
suspected ectopic pregnancy
◾ Vaginal bleeding (in all trimesters)
◾ Fetal anomalies (anencephaly)
◾ Suspected molar pregnancy
◾ Gestational age
◾ Multiple pregnancies (chorionicity)
◾ To confirm cardiac activity
◾ Screening of aneuploidy
◾ Evaluation of pelvic/adnexal masses (all trimesters)

Mid-trimester ultrasound 

A routine mid-trimester ultrasound scan is often performed between dating the pregnancy (more accurate if established earlier). And the timely detection of major congenital anomalies.

Major points to keep in mind during Mid-trimester ultrasound

◾ Fetal growth
◾ Gestational age assessment
◾ Important observation
◾ Fetal Heart
◾ Fetal abdomen and abdominal wall
◾ Placenta and umbilical cord
◾ Placenta of multifetal pregnancy (chorionicity)
◾ Fetal weight.

What basis fetal growth is calculated?

Fetal growth is calculated based on an accurate gestational age and is expressed in percentiles.

Normal fetal weight should be between the 10th and 90th percentiles. Weightless than 10th percentiles are considered small for gestational age (SGA). Whereas more than the 90th percentile is large for gestational age.

Based on biometric data, computer software can calculate fetal weight using Hadlock's Formula

What is gestational age assessment and what are the parameters?

Nearly 20% of pregnant women are uncertain about the last menstrual period.

In the second trimester, an optimum time for the most accurate assessment of gestational age is between 14 weeks and 20 weeks.

Parameters: The parameters used are biparietal diameter (BPD), head circumference (HC) abdominal circumference (AC), and femur length (FL).

How HC, BPD, AC, FL is recorded (measured)?

  • The fetal head (HC)  is imaged in a transverse axial section at the level flex cerebri, cavum septum pallidum, and the thalamic nuclei.
  • BPD: is recorded from the outer skull edge of the proximal skull to the inner edge of the proximal skull.
  • AC: is measured preferably at the level of the junction of the left and right portal veins, umbilical cord vein, and as round as possible.
  • FL: is measured when the beam from the transducer is perpendicular to the shaft.
AC is the single most sensitive parameter for the assessment of fetal growth.

Transcerebellar diameter (TCD) 

TCD is an accurate predictor of gestational age when done between 14 weeks and 28 weeks. It is rarely affected by fetal growth aberrations.

Averaging the measurements of BPD, HC, AC, and FL

The variation  of estimated gestational age to true gestational age is:

  • In the first trimester: When the LMP and US-based dating difference is >- 7 days, it is better to accept a US-based date.
  • In the second trimester (13-18 weeks): The difference may be between 10 days and 14 days.
  • In the third trimester (29-42 weeks): The difference may be >- 21 days.
Dating ultrasound done before 22 weeks should be used in preference to menstrual dates irrespective of the reliability or closeness with menstrual dates.

An important observation in obstetrics ultrasonography

There is three important observation that must be observed by a sonographic technician.
  1. Cranial abnormalities
  2. Fetal face
  3. Spinal anomalies
  4. Fetal heart
  5. Fetal abdomen and abdominal wall.
1. Cranial abnormalities

In cranial abnormalities the five major points to keep in mind during the second-trimester ultrasound.
  • Obliteration of cisterna magna.
  • Compression of flattering of the posterior cerebellar surface making a crescentic appearance (banana sign).
  • Concave deformity of the frontal bones (lemon sign).
  • The absence of a calvarium is also called anencephaly.
  • Hydrocephaly may be suspected when the HC or BPD is enlarged. Ventriculomegaly is diagnosed when the width of the lateral ventricle is >10mm and the choroid plexus is seen dangling.

Fetal face- for cleft lip or plate

(A) Anencephaly is diagnosed by the absence of cranial vault (calvarium) and telencephalon. Brain tissue is angiomatous. Early diagnosis is possible at about 13 weeks. Encephalocele is the protrusion of the brain and/or meninges through a cranial defect.

(B) Spinal anomalies
Spinal anomalies may be of two types:
  1. Spina bifida occulta is categorized by a vertebral schisis covered by normal soft tissue.
  2. Spina bifida aperta is a full-thickness defect of the skin and vertebral arches. The neural canal is exposed. This defect may be covered by thin meninges (meningocele) and with neural tissue (myelomeningocele).

Screening for fetal heart

When screening a fetal heart four-chamber view of the heart and evaluation of outflow tracts are done for viewing of congenital heart disease. In case of abnormalities, fetal echocardiography is more informative.

Fetal abdomen and abdominal wall

  • Hyper echoic bowel
  • Fetal kidney
  • Omphalocele and gastroschisis

In the fetal abdomen and abdominal wall, a stomach bubble is seen normally by 20 weeks of gestational age. Its absence suggests a fetal anomaly known as esophageal atresia.

In the fetal abdomen hyperechoic bowel has been associated with chromosomal abnormalities. Kidneys are usually hyperechoic. Also screening the fetal kidney for any obstructive or dysplastic abnormalities.

Differentiation of abdominal wall defects

Omphalocele and gastroschisis are rare ( 1 in 4,000 live births) with abdominal wall defects.
Abdominal wall defect

  • Usually midline defect
  • Cord insertion is on the herited mass
  • Hernia contains intestines, liver or both
  • Covered by a membrane (peritoneum and amnio)
  • Associated chromosomal anomaly - 40-60%
  • Prognosis- often poor
  • Paraumblical defect
  • Usually on the right side of the cord insertion
  • Herniated small bowel loops floating free in amniotic fluid
  • Not covered by any membrane
  • Not associated with a chromosomal anomaly
  • Prognosis- good

What confirms the fetal gender (identification)?

The fetal gender identification is confirmed by the detection of testes within the scrotum in the third trimester. Fetal perineal examination for external genitalia may be incorrect in the second trimester in 1% of the cases.

Placenta and umbilical cord

The placenta is an echogenic discoid mass. Placental thickness at term is about 30mm. Placental thickness more than 45 mm at any period of gestation is considered abnormal.

The relationship of the placenta to the internal cervical OS is important to define low-lying placenta and placenta previa false-positive diagnosis may be due to focal uterine contraction or material bladder (to full/too empty). Transperineal or transvaginal imaging may be necessary in that case.

When the distance between the internal OS and placental edge is more than 20 mm, placenta previa is allowed. Only 5% of placenta previa identified in the second trimester will persist to term.

Umbilical cord

Single umbilical artery (SUA) is associated with a higher rate of fetal anomalies (30-70%).
The three major anomalies observed are:
  1. Cardiac
  2. CNS
  3. Kidneys
velamentous insertion and vasa previa can be diagnosed with Doppler ultrasound.

Placenta of multifetal pregnancy (chorionicity)

Dizygotic twins have always diamniotic- dichorionic (DiDi) placenta, whereas DiDi may be absorbed in 20-30% of monozygotic twins. The proliferating placental villi grow into the interchronic space of the two placentate. This projected placental tissue is shaped like a triangle and has the same echogenicity as that of the placenta. The base of the triangle is toward the chronic surface and the apex is toward the dividing membrane. 

Twin peak sign

Twin peak sign is neither present entirely over the placenta nor is it a consistent feature.
     The diagnosis of DiDi twins is made when the thickness of the dividing membrane is more than 2 mm with the progress of pregnancy, the membrane becomes attenuated.

What suggests monochorionic diamniotic twin pregnancy?

The presence of one gestational sac with a thin (<2 mm), dividing membrane and two fetuses (T sign), suggests monochorionic diamniotic twin pregnancy.

Fetal weight

Hardlock formula is used.
Hardlock formula is sensitive to 7.5% of the actual fetal weight using fetal biometric data (BPD, HC, AC, and FL).

For fetal weight commonly used four variables:
  1. BPD
  2. HC
  3. AC
  4. FL
The absolute error for birth weight prediction is about 8-10%. ultrasound and clinical examinations have similar accuracy for predicting birth weight.

Third-trimester ultrasonography

In the third trimester ultrasound, all the information of the second trimester ultrasound is obtained.
  • A detailed anatomical survey should be done now even if the previous survey was normal. Achondroplastic dwarfish is diagnosed in the third-trimester USG.
  • Estimated fetal weight (EFW) is determined from the average of three readings for each of these: FL, AC, and BPD. AC is the most important sonographic EFW has an error risk of 15-20%.

What gestation effects at 18-22 weeks of routine USG?

The gestation effects at 18-22 weeks are as follow:
  • Reduces the incidence of post-term pregnancy (39%) and rates of induction of labor for post-term pregnancy
  • Increases detection of multiple pregnancies (92%)
  • Increases detection of major fetal anomalies when termination is possible
  • No significant differences in the clinical outcomes such as perinatal mortality
  • Reduces neonatal admission to special care baby unit (14%).

Yolk sac

The yolk sac is the first element seen within the gestational sac during pregnancy, usually at 3days gestation. The yolk sac is situated on the front (ventral) part of the embryo; it is lined by extra-embryonic endoderm, outside of which is a layer of extra-embryonic.

What is the yolk sac?

In early pregnancy, the yolk sac functions as a source of nourishment for the developing fetus. The first structure is visible within the gestational sac, which envelopes the developing fetus and the amniotic fluid.

What is the function of the yolk sac?

The yolk sac functions as an absorptive for nutrient uptake and secretion as well as the origin of the first blood cells. In human and non-human primates, the allantois is a small diverticulum, which is part of the umbilical cord, connects to the bladder, and acts as a temporary store for foetal excretions.

Can you have a yolk sac and no baby?

It contains a yolk sac (protruding from its lower part) but no embryo, even after scanning across all planes of the gestational sac, thus being diagnostic of an embryonic gestation. A blighted ovum is a pregnancy in which the embryo never develops or develops and is reabsorbed.

Does a yolk sac have a heartbeat?

Every patient with accurate dates greater than 40 days had an embryo with a heartbeat identified. When correlating sac size with structures within the sac, a yolk sac was first seen in a gestational sac between 6 and 9mm, and a heartbeat is seen in every patient with a 9-mm or greater gestational sac diameter.

What is the difference between the yolk sac and the fetal pole?

A yolk sac is usually seen by 6 menstrual weeks, or by the time the mean diameter of the sac has reached 10 mm. A fetal pole with heart tones is typically seen by the completion of 7 menstrual weeks.

When the yolk sac is seen in ultrasound?

From about 7 weeks onwards, it is usually possible to see a round cystic structure about 4-5 mm in diameter adjacent to the fetus. This is the yolk sac, the site of the earliest blood cell formating. It disappears at about the eleventh week. The yolk sac may not be seen in all pregnancies, even when quite normal.

It is important to recognize that cystic shadow is the yolk sac and not mistake it for a second, twin embryo. (Thre yolk sac is not included in crown-rump measurements.


An embryo is the initial stage of development of a multicellular organism. In general, in organisms that reproduce sexually, embryonic development is part of the life cycle that begins immediately after fertilization and continues through the formation of body structures, such as tissues and organs.

When does the embryo become visible?

Although the gestational sac can be recognized at 5weeks in some patients and at 6 weeks in the majority, the embryo does not become visible until the eighth gestational week. It will then be shown as a focal area of echoes, often lying eccentrically within the gestational, sac. If the fetus is alive, the heart will be recognized lying in mid-embryo, usually seeming to lie anterior to the rest of the thorax.

After the ninth month and tenth week, the foetal head can be distinguished from the body and movements can be seen. At 10 weeks the fetus becomes more human in appearance. After the twelfth week, the skull becomes visible.

What is called an embryo?

Embryo, the early developmental stage of an animal while it is in the egg or within the uterus of the mother. In humans the term is applied to the unborn child until the end of the seventh week following conception; from the eighth week, the unborn child is called a fetus.

What is the embryonic period?

The process of prenatal development occurs in three main stages. The first two weeks after conception are known as the germinal stage, the third through the eighth weeks are known as the embryonic period, and the time from the ninth week until birth is known as the fetal period (embryonic period).

Is an embryo life?

Embryos are whole beings, at the early stage of their maturation. The term 'embryo', Similar to the term maturation. The term 'infant' and 'adolescent' refers to a determinate and enduring organism at a particular stage of development.

What is the difference between a foetus and an embryo?

An embryo is formed by the repeated cell division of a zygote.

A foetus is formed by the growth and development of an embryo.

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