Obstetric Ultrasonography (BMRC – CMU Exam Preparation)

In a clinically normal pregnancy, ultrasound is done in two stages

  • At 18 – 22 weeks after the first day of the women’s last menstrual period
  • At 32 – 36 weeks after the first day of the women’s last menstrual period
Ultrasound is thought to be safe during pregnancy even though scans are conducted only when there is good clinical reason 

Trimesters Of Pregnancy

First trimester : 1 – 12 weeks of pregnancy

Second trimester : 13 – 28 weeks of pregnancy
Third trimester : 29 – 40 weeks of pregnancy

5th or 6th week is the earliest stage at which a pregnancy can be recognized by ultrasound

Importance Of Ultrasound Scans During First Trimester
  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 a family history is an entirely reliable way to detect multiple pregnancy, 
  4. A significant number of mothers with a low-lying placenta (placenta praevla) show no evidence until bleeding starts at the onset of labour
  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. 

Sex determination is not an indication for ultrasound, unless the pregnancy is associated with a sex linked disorder

Indications Of Ultrasound Scan In First Trimester

  1. To confirm the presence of an intrauterine pregnancy
  2.  To evaluate a suspected ectopic pregnancy
  3. To find the cause of per-vaginal bleeding 
  4. To evaluate pelvic pain 
  5. To estimate the gestational age
  6. To confirm fetal cardiac activity 
  7. To assess fetal anomalies – Anencephaly 
  8. To evaluate maternal pelvic masses and / or uterine abnormalities
  9. To evaluate suspected molar pregnancy 
Indications Of Ultrasound Scan In Second And Third Trimester 
  1. To estimate gestational age 
  2. To evaluate fetal growth
  3. To evaluate fetal well being
  4. To evaluate per-vaginal bleeding
  5. To evaluate pelvic pain
  6. To determine the fetal presentation and position
  7. In suspected multiple pregnancy
  8. Suspected IUD, IUGR
  9. Suspected amniotic fluid abnormalities
  10. Suspected abruptio placenta 
  11. Follow up of fetal anomalies, placenta previa 
  12. Adjunct to amniocentesis, external cephalic version 

Information Obtained From Ultrasound Scans In Each Trimesters

First Trimester

  1. To confirm pregnancy
  2. To estimate gestational age 
  3. To diagnose ectopic pregnancy
  4. To recognize single or multiple pregnancy
  5. To exclude molar pregnancy, pseudo-pregnancy due to a pelvic mass or hormone-secreting ovarian tumour
  6. To diagnose myomas or ovarian masses which might interfere with normal delivery 

Second Trimester / (18 – 22 Weeks Scan)

  1. To establish the gestational age accurately 
  2. To diagnose multiple pregnancy
  3. To diagnose fetal abnormalities
  4. To locate the placenta and identify patients in whom there is a risk of placenta praevia
  5. To recognize myomas or any other unexpected pelvic mass that may interfere with pregnancy or delivery 

Third Trimester / 32 – 36 Weeks Scan 

  1. To diagnose IUGR 
  2. To recognize fetal anomalies that were not detected at the first scan
  3. To confirm the presentation and position of the fetus
  4. To locate the placenta accurately
  5. Assess the amount of amniotic fluid / AFI 
  6. To exclude possible complications (myoma, ovarian tumour) 

Preparation Of The Patient For Obstetric Scan 

  1. Bladder must be full
  2. 4 – 5 glasses of water is given, one hour prior to examination and patient is not allowed to micturate 
  • Alternate method of filling is by a urethral catheter by normal saline 
  • Filling is done till the patient feels uncomfortable 
  • This method is avoided as there is chance of infection 
Key Measurements Of Obstetric Ultrasound In Each Trimester
First Trimester
  1. Location of gestational sac
  2. Gestational age 
    • Mean sac diameter
    • Embryonic pole length 
    • Crown rump length 
  3. Yolk sac or embryo or fetus 
  4. Cardiac activity
  5. Number of fetuses (Amnionicity / chorionicity)
  6. Maternal anatomy (Uterus and adnexa)
Second And Third Trimester
  1. Cardiac activity
  2. Presentation 
  3. Number of fetuses 
  4. Gestational age 
    • Biparietal diameter
    • Head circumference 
    • Abdominal circumference 
    • Femoral length 
  5. Amniotic fluid – AFI
  6. Position of placenta 
  7. Maternal anatomy (Uterus and adnexa)
  8. Fetal anatomy
    • Head, face, neck 
    • Chest
    • Abdomen 
    • Spine
    • Extremities 
    • Gender 
Gestational Sac

First have to localize the gestational sac – Intrauterine or extrauterine
Can be recognized after 6 weeks of pregnancy

  1. The centre is hypoechogenic
  2. Centre is surrounded by a double echogenic ring 
  3. Inner ring is complete and has a uniform echogenicity with a thickness of 2 mm or more
  4. Outer ring is incomplete  
  5. Between the two rings, there is an anechoic residual uterine cavity 
Fig : Gestational Sac

The Embryo 
Seen at 8 weeks of pregnancy 
  1. Focal area of echoes 
  2. Lies eccentrically within the gestational sac
  3. If alive, the heart can be recognized lying in the mid embryo and anterior to the rest of the thorax 
Fig : Embryo With Decidual Reaction


The Yolk Sac 
Present after 7 weeks of pregnancy till 11 weeks of pregnancy 
It will not be present in all pregnancies 
It is the earliest site of blood cell formation
Yolk sac is not included in the crown rump measurement  
  1. Round cystic structure about 4 – 5 mm in diameter
  2. Present adjacent to the fetus 
Fig : Yolk Sac

After 9th or 10th week of pregnancy – Can distinguish between head and body and movements can be seen 
After 10th week of pregnancy – Fetus become more human in appearance 
After 12th week of pregnancy – Skull becomes visible 
Blighted Ovum / Anembryonic Pregnancy
Anembryonic pregnancy is a form of a failed early pregnancy, where a gestational sac develops, but the embryo does not form
  1. May be asymptomatic, diagnosed during early pregnancy ultrasound
  2. Per-vaginal bleeding in early pregnancy
  3. Subsidence of pregnancy signs and symptoms
Sonographic findings
  1. Small gestational sac
  2. Absence of an embryo
  3. If this is seen in early pregnancy scan, re-examination after 7 days 
    • In case of normal pregnancy, sac will grow, fetus and cardiac activity will be present 

Fig : Blighted Ovum


Fetal Biometry – Estimate Fetal Size And Gestational Age
Mean Gestational Sac Dimension
Gestational age of the fetus can be estimated by reference to local standard development tables
Three measurements are required to calculate mean gestational sac dimension

  • In longitudinal scan 
    • Length
    • Antero-posterior dimeter
  • In transverse scan 
    • Width 


Fig : Mean Gestational Sac Dimension

Crown-rump length (CRL)

Most reliable parameter for estimating gestational age from 7 weeks – 11 weeks of pregnancy 
Longest length of the embryo is taken to take the measurement 
Measurement should not include the yolk sac or fetal limbs

Fig : CRL

Biparietal diameter (BPD)
Most reliable parameter for estimating gestational age between 12 weeks – 26 weeks of pregnancy
It is the distance between the two parietal eminence 
In a transverse scan of fetal skull with falx cerebri in the midline interrupted by thalami and cavum septi pellucidi 
Measurement is taken from the outer table of the proximal skull to the inner table of the distal skull 
(Leading edge to leading edge technique)

Fig : BPD

Fronto-occipital diameter
Longest axis from outer edge to outer edge 
Measured at the same level as BPD
Cephalic index
Used when the shape of the head is abnormal or when there is an abnormality in the intracranial contents


Head circumference
If cephalic index is outside the range, BPD is not used to determine the gestational age
Instead of BPD, head circumference is used


Abdominal circumference
Used to detect intrauterine growth disturbances 
Measured at the level of the fetal liver
Scan should include stomach and umbilical portion of left portal vein 
Left portal vein should be short, not elongated 
Antero-posterior diameter and tranverse diameter is required to make the calculation
Fig : AC At The Level Of Liver




Femoral length
Fetal long bones are seen after 13 weeks of pregnancy 
Measurement is obtained from one end to the other end
Longest length of the bone should be obtained
This is used to estimate gestational age when intracranial pathology is detected
Fig : Femoral Length

Multiple Pregnancy
When more than one fetus simultaneously develops in the uterus is called as multiple pregnancy 
Types Of Multiple Pregnancy
According to the number
  1. Twins 
  2. Triplets 
  3. Quadruplets 
  4. Quintuplets 
  5. Sextuplets 

According to the zygosity

  1. Dizygotic twins / Biovular – 80%  
  2. Monozygotic twins / Uniovular – 20%

According to the chorionicity

  1. Diamniotic-dichorionic or D/D  (30%)
    • Division takes place within 72 hours after fertilization (prior to morula stage)
    • Resulting embryos will have two separate placenta, chorions and amnions
  2. Diamniotic monochorionic  or D/M  (66%)
    • Division takes place between the 4th and 8th day 
    • (After the formation of inner cell mass when chorion has already developed)
  3. Monoamniotic-monochorionic twin develops or M/M  (3%)
    • Division occurs after 8th day of fertilization 
    • Then the amniotic cavity has already formed
  4. Conjoined twins / Siamese twin 
    • Division occurs after 2 weeks of the development of embryonic disc
    • Results in the formation of conjoined twin 


Causes / Risk Factors For Multiple Pregnancy
  1. Ovulation inducing drugs 
  2. Assisted reproductive techniques 
  3. Age more than 35 years
  4. Grandmultiparity 
  5. Family history multiple pregnancy  
  6. Race – Highest among negros 
  7. Idiopathic
  1. History of use of ovulation inducing drugs, ART, positive family history
  2. Exaggerated pregnancy signs and symptoms – Increased nausea and vomiting 
  3. Cardiopulmonary embarrassment – Palpitations, dyspnoea 
  4. Unusual rate of abdominal enlargement 
  5. Excessive fetal movements 
  6. May be anaemic 
  7. Unsual weight gain 
  8. Abdominal examination 
    • Inspection 
      1. Barrel shaped and distended abdomen 
    • Palpation 
      1. Height of the uterus is more than the period of amenorrhoea 
      2. More than two fetal pole can be felt 
      3. More fetal parts can be felt 
      4. Abdominal girth at term is more than normal (Normal – 100cm) 
    • Auscultation
      1. FHR – Auscultated by two observers has a difference of at least 10 beats/min
        • Simultaneous hearing of two distinct fetal heart sound located at separate spots with a silent area in between by two observers 
Sonographic Findings
  1. More than one fetus is seen
  2. More than one placenta maybe seen 
  3. Separated by a membrane which is hypoechogenic 


Fig : Multiple Pregnancy

Interruption of the conception of pregnancy before 28th weeks of pregnancy with or without the expulsion of fetus either spontaneously or induced is called abortion.
Classification Of Abortion
  • Spontaneous
    1. Threatened abortion
    2. Inevitable abortion
    3. Complete abortion
    4. Incomplete abortion 
    5. Missed abortion
    6. Septic abortion
  • Induced
    1. Legal 
    2. Illegal 
Incomplete Abortion
When some products of conception are expelled out and some are retained inside the uterine cavity is called incomplete abortion. 
  1. History of variable period of amenorrhoea
  2. Sudden severe lower abdominal pain
  3. Profuse per-vagina bleeding 
  4. Expulsion of a fleshy mass per vagina, followed by….
    • Continuance of pain in the lower abdomen which is colicky in nature by in a diminished magnitude 
    • Persistent per-vaginal discharge of varying magnitude 
  5. Anaemia 
  6. Signs of shock (Low BP, Cold clammy skin, Rapid weak pulse, Dizziness, Fainting)
  7. On per-abdominal examination
    • Height of the uterus is smaller than the period of amenorrhoea
  8. On per-vaginal examination
    • Uterus is soft
    • Internal OS is open
    • Products of conception is felt on the cervical canal / vagina
Sonographic Findings 
  1. Height of the uterus is smaller than expected for the gestational age 
  2. Abnormal shaped sac or an amorphous mass of variable size shape and echogenicity 
  3. No fetal cardiac activity
Fig : Incomplete Abortion


Sonographic Difference Between Threatened Abortion And Missed Abortion
Threatened Abortion
Missed Abortion
1. Height of uterus Corresponds to the period of amenorrhoea Smaller than the period of amenorrhoea
2. Fetal heart rate and movements Present, if developed Absent
Hydatidiform Mole / Molar Pregnancy 
It is an abnormal trophoblastic proliferation associated with hydropic degeneration 
  1. Variable period of amenorrhoea
  2. Lower abdominal pain
  3. Recurrent per-vaginal brown stained discharge
  4. Expulsion of grape like vesicles from vagina
  5. Exaggerated early pregnancy symptoms such as nausea and vomiting
  6. Constitutional symptoms – Breathlessness 
  7. Cachexia 
  8. Anaemia 
  9. Blood pressure  : Variable 
  10. Per-abdominal examination
    • Symphysio-fundal height is more than the period of gestation
    • Doughy feeling
    • No fetal parts are palpable 
    • No audible fetal heart sound
  11. Per-vaginal examination
    • Soft more than the period of gestation
    • Passage of grape like vesicles
    • Uterus is flabby
    • Luteal cyst at the fornix 
    • Bilateral enlargement of the ovaries – Theca lutein cyst 
Sonographic findings
  1. Uterus is enlarged
  2. Uterus is filled with an uniform echogenic mass giving a regular speckled appearance – “Snow storm effect”
  3. Has a strong back wall echoes
  4. Central necrosis
  5. Fetus or embryo may be present 
Fig : Molar Pregnancy
Intrauterine Growth Retardation (IUGR)
Intrauterine growth restriction is said to be present in those babies whose birth weight is below the 10th  percentile of the average for the gestational age, in a community
Types Of IUGR
  1. Constitutional IUGR
  2. True IUGR
    • Type I / Symmetrical IUGR : 20%
    • Type II / Asymmetrical IUGR : 80% 
Causes Of IUGR
  • Maternal 
    • Constitutional
      1. Small women
      2. Maternal genetics 
      3. Racial background
    • Maternal under nutrition before and during pregnancy
    • Maternal diseases
      1. Anemia
      2. Hypertension
      3. Thrombotic diseases
      4. Heart disease
      5. Chronic renal disease
      6. Collagen vascular disease
    • Toxins
      1. Alcohol
      2. Smoking
      3. Cocaine
      4. Heroin
  • Fetal causes
    • Structural anomalies
      1. Cardiovascular defects
      2. Renal defects
    • Chromosomal abnormality
      1. Triploidy
      2. Turner’s syndrome
      3. Polyploidy
    • Infection 
      1. TORCH agents
      2. Malaria
    • Multiple pregnancy (Mechanical hindrance to growth and excessive fetal demand)
  • Placental causes
    1. Placenta previa
    2. Abruption
    3. Circumvallate
    4. Infarction
  • Unknown (40%)
  1. Maternal weight remains stationary or falling 
  2. Symphysio-fundal height has a lag of 4 cm
  3. Abdominal circumference is stationary or falling
Sonographic findings / Difference Between Type 1 And Type 2 IUGR
Symmetrical IUGR
Asymmetrical IUGR
1. Head : Body ratio Normal Abnormal
2. Onset Starts early in pregnancy Starts late in pregnancy
3. Measurements All measurements are reduced equally Abdominal circumference is less than normal
4. Size of fetus Uniformly small Head is larger than abdomen
5. Aetiology
Genetic disease or infection

(Intrinsic to fetus)

Chronic placental insufficiency

(Extrinsic to fetus)

6. Affect on fetus Fetus is affected from the noxious effect very early in the phase of cellular hyperplasia Fetus is affected in later months during the phase of cellular hypertrophy
Intrauterine Fetal Death (IUD)
Antepartum death occurring beyond 28 weeks of pregnancy is termed as intrauterine fetal death (IUD)
Causes Of Intrauterine Fetal Death
  • Maternal causes
    1. Hypertension 
    2. Pre-eclampsia 
    3. Eclampsia
    4. Chronic hypertension
    5. Diabetes mellitus
    6. Maternal infection
      • Malaria
      • Toxoplasmosis
      • Influenza
      • Syphilus 
    7. Hyperpyrexia 
    8. Anti-phospholipid syndrome
    9. Systemic lupus erythematosis (SLE)
  • Fetal causes
    1. Chromosomal abnormalities
    2. Major structural anomalies
    3. Fetal infection
      • Chorioamniotits 
      • Rubella
      • CMV
    4. Rh incompatibility 
    5. IUGR
  • Placental causes
    1. Antepartum haemorrhage (Abruptio placenta, placenta previa)
    2. Twin-to-twin transfusion syndrome (TTTS)
    3. Prolapse of the cord
    4. Twisting of the cord
    5. Placental insufficiency
  • Iatrogenic (25 – 35%)
    1. External cephalic version
    2. Drugs (Quinine)
  1. Absence fetal movement 
  2. Retrogression of pregnancy signs
  3. Symphysio-fundal height is less than the period of amenorrhoea
  4. Uterine tone is diminished and uterus become flaccid
  5. Fetal heart sounds are absent
  6. Fetal movement is absent
Sonographic findings
  1. No fetal cardiac activity after careful observation for several minutes
  2. Oligohydromnios
  3. Collapse of the skull with overriding of the skull bones (Spanlding’s sign)
Fig : IUD

Placenta becomes easier to locate after 14 weeks of pregnancy 
Normal site of placenta
The placenta is usually attached to the upper part of the body of the uterus encroaching to the fundus adjacent to the anterior or posterior wall with equal frequency 
Positions of placenta
It is described in relation to the wall of uterus and cervical os
Right, midline, left ??
Anterior, anterofundal, fundal, posterofundal, posterior ??
Placenta Previa
When the placenta is implanted partially or completely over the lower uterine segment (over and adjacent to the internal os) it is called placenta previa
Types of placenta previa
  1. Central placenta previa
    • Placenta covers the os completely 
  2. Marginal placenta previa
    • Edge of the placenta covers the os 
  3. Low lying placenta
    • Lower edge of the placenta lies close to the os 
Predisposing Factors Of Placenta Previa
  1. Multiparity
  2. Increased maternal age (More than 35 years)
  3. History of previous cesarean section or any other scar in the uterus (myomectomy or hysterotomy)
  4. Placental size (mentioned before) and abnormality (succenturiate lobes) 
  5. Smoking (Causes placental hypertrophy to compensate carbon monoxide induced hypoxemia)
  6. Prior curettage
  1. Sudden onset of painless bleeding which is apparently causeless
  2. Recurrent bleeding
  3. Varying degree of anaemia depending on the amount of blood loss
  4. Shows signs of shock (Hypotension, tachycardia, cold clammy skin, tachypnoea)
  5. Patient appears exhausted 
  6. On per-abdominal examination
    • Symphysio-fundal height corresponds with the period of amenorrhoea 
    • Abdominal wall is relaxed, soft and elastic
    • No tenderness present
    • Abnormal presentation is common
    • Fetal head is found floating in contrast to the period of gestation 
    • FHR : Usually present
  7. On per-vaginal examination (CONTRAINDICATED)
    • Only inspection is done
    • Bright red bleeding is seen 
Sonographic findings
To confirm placenta previa, it should be scanned with a full bladder and rescanned with a partially emptied bladder 
Fig : Placenta Previa

Congenital Fetal Anomalies
  1. Neurological anomalies 
    • Anencephaly 
    • Hydrocephaly 
    • Microcephaly 
    • Encephalocele 
  2. Spinal anomalies 
    • Myelomeningocele 
    • Spina bifida 
  3. Cystic hygroma 
  4. Cardiac anomalies 
    • Malposition 
    • Ventricular septal defect 
    • Hypoplasia 
  5. Gastrointestinal anomalies 
    • Duodenal atresia 
    • Jejunal atresia 
    • Cardiac atresia 
  6. Abdominal wall defects 
    • Omphalocele 
    • Gastroschisis 
    • Fetal ascites 
  7. Renal anomalies 
    • Hypoplasia 
    • Obstruction 
    • Cystic disease 
  8. Amniotic fluid 
    • Oligohydramnios 
    • Polyhydramnios 
  9. Fetal death 
It is the absence of the cranial vault and the cerebral hemisphere due to defects in the development of neural tube 
The incidence of anencephaly is about 1 in 1000 births
Can be diagnosed by 12 weeks of pregnancy by ultrasonography 
Increased alpha fetoprotein 
Absence of vault of skull and brain

Fig : Anencephaly


Hydrocephalus refers to an increase in CSF volume and ventricular enlargement due to disturbance of production, flow or reabsorption of CSF.
Can be recognized at 18 weeks of pregnancy
Dilatation of the anterior and posterior horns of the lateral ventricles 
Fig : Hydrocephalus
Amniotic Fluid Index
The amniotic fluid index (AFI) is an estimate of the amniotic fluid volume in a fetus. It is part of the fetal biophysical profile.
Normal value is 8 – 24 cm
Less than 5 cm is considered to be oligohydrominos
More than 24 cm is considered to be polyhydromnios
Uterus is divided into four imaginary quadrants with linea nigra and umbilicus acting as the vertical and the horizontal axis respectively
For the measurement fetal parts are avoided and probe is placed vertically 
Sum of all the four quadrant measurements is AFI
Fig : Quadrant Of Uterus To Measure AFI
Clinically polyhydromnios is defined as the excessive accumulation of liquor amnii causing discomfort to the patient and/or when an imaging help is needed to substantiate the clinical diagnosis of the lie and presentation of the fetus 
Causes Of Polyhydromnios
  1. Fetal congenital anomalies
    • Anencephaly
    • Open spina bifida
    • Oesophageal or duodenal atresia
    • Facial clefts and neck masses
    • Hydrops fetalis
    • Aneuploidy
  2. Placental causes
    • Chorioangioma of the placenta
  3. Multiple pregnancy 
  4. Maternal 
    • Diabetes
    • Cardiac or renal disease
  5. Idiopathic
  1. Breathlessness
  2. Palpitations
  3. Swelling of the legs 
  4. Varicosities of the legs or vulva 
  5. Per-abdominal examination 
    • Inspection 
      • Markedly enlarged, globular in shape
      • Fullness at the flanks
      • The skin is tense, shiny with large striae
    • Palpation
      • Height of the uterus is more than the period of amenorrhea
      • Abdominal girth is more than normal
      • Fluid thrill present
      • Fetal parts cannot be felt properly 
      • External ballottement can be elicited easily
    • Auscultation
      • Fetal heart sound is not heard distinctly
  6. Per-vaginal examination 
    • The cervix is pulled up, may be partially taken up or at times, dilated, to admit a finger tip through which tense bulged membranes can be felt
Sonographic findings
  1. Amniotic fluid index (AFI) is more than 24 cm
  2. Largest vertical pocket is more than 8 cm

It is an extremely rare condition where the liquor amnii is deficient in amount to the extent of less than 200mL at term
Causes of oligohydromnios
  • Fetal conditions
    1. Fetal chromosomal or structural anomalies
    2. Renal agenesis 
    3. Obstructed uropathy 
    4. Spontaneous rupture of the membrane
    5. Intrauterine infection
    6. Drugs: PG inhibitors, ACEinhibitors 
    7. Postmaturity
    8. IUGR
    9. Amnion nodosum (failure of secretion by the cells of the amnion covering the placenta)
  • Maternal conditions
    1. Hypertensive disorders 
    2. Uteroplacental insufficiency
    3. Dehydration
    4. Idiopathic
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