Appearances of Organs and Body Parts in Pregnancy

1. Uterus

• Weeks 1-12: Remains entirely within the bony pelvis; an intrapelvic organ
• Week 12: Begins to rise out of the pelvis, becomes palpable abdominally
• Week 20: Reaches the umbilicus
• Weeks 34-36: Reaches the costal margins
• At term: Enlarged by ~30 cm and increased 15-fold in weight

• Hegar's sign (8-10 weeks): The isthmus (between the body and cervix) feels soft and compressible on bimanual exam; the cervix feels firm and the body feels elastic
• Braxton Hicks contractions: Intermittent, painless contractions from the 3rd-4th month onward
• Red, subcutaneous scars (striae gravidarum): Appear on the abdomen due to skin stretching

2. Cervix

• Softens progressively from below upward from the 2nd month onward - this is Goodell's sign, well marked by the 4th month
• At the last month: the cervical os becomes circular (instead of transverse) and admits a fingertip more easily
• At 3rd month: the angle between the body and cervix is accentuated

3. Vagina

• Mucous membrane changes color: pink → violet → blue due to venous congestion
• Visible from the 4th week onward
• Known as Chadwick's sign (or Jacquemier's sign)
• Tissue becomes softer, secretions increase, and pulsations of the vaginal artery become palpable early

4. Breasts

• Size increases due to hypertrophy of mammary glands
• Superficial veins become engorged and visible
• Nipple becomes deeply pigmented and erectile
• Areola becomes darkly brown by the 2nd month (Montgomery's tubercles - sebaceous glands enlarge and form tubercles around the nipple)
• Colostrum formation begins in the 3rd month (thin yellowish fluid)
• Silvery striae appear on the breast skin after 6 months from stretching

5. Skin

• Skin of the vulva, abdomen, and axilla become darker (hyperpigmented)
• Dark circles form around the eyes (chloasma/melasma)
• Linea nigra forms: a dark, pigmented line running from the pubis, encircling the umbilicus, and continuing up to the ensiform (xiphoid) cartilage
• Striae gravidarum (stretch marks): red/silvery lines on the abdomen and breasts

6. Diaphragm and Chest

• The diaphragm progressively rises as the uterus enlarges
• Compensatory flaring of the ribs occurs
• On a chest X-ray: the heart appears enlarged (shifted position), and prominent vascular markings are visible due to increased pulmonary blood volume
• On ECG: left axis deviation and T-wave changes are seen
• This elevated diaphragm means thoracostomy in the third trimester requires chest tube placement one to two interspaces higher than the usual 5th interspace

7. Heart

• The heart is displaced upward and to the left by the rising diaphragm
• On physical exam: a grade I-II systolic ejection murmur is common (flow murmur from increased cardiac output)
• Exaggerated splitting of S1; an S3 may be audible
• A small, asymptomatic pericardial effusion may develop
• Blood volume increases ~35%; cardiac output rises ~40%

8. Abdominal Viscera (Bowel, Stomach, Appendix)

• All abdominal organs are pushed upward by the enlarging uterus
• The stomach is displaced upward and anteriorly, promoting gastroesophageal sphincter incompetence - hence heartburn and reflux are common
• The cecum and appendix are displaced upward - right lower quadrant pain shifts toward the right upper quadrant as pregnancy advances, complicating diagnosis of appendicitis
• The normal peritoneal response (guarding, rebound) is blunted because the abdominal wall is stretched - injury severity can be underestimated

9. Kidneys and Ureters

• Kidneys increase in size by approximately 1 cm in length (measured by ultrasound)
• Dilation of the collecting system (hydronephrosis/hydroureter) begins as early as the 3rd month - the right renal pelvis is most often affected
• This dilation is partly hormonal (not just mechanical compression) and normally resolves by 12 weeks postpartum
• Persistent dilation beyond 12-16 weeks postpartum suggests true mechanical obstruction
• Ureteral dilation from smooth muscle relaxation or compression by the gravid uterus is a normal finding on imaging

10. Bladder

• In the first trimester: the bladder is protected by the bony pelvis
• After the 3rd month: both the uterus and bladder rise out of the pelvis, becoming vulnerable to direct injury
• Both organs become hyperemic (engorged with blood) during pregnancy
• The enlarging uterus presses on the bladder, causing urinary frequency - this symptom eases mid-pregnancy and returns when the fetal head descends into the pelvis near term

11. Epidural Venous Plexus and Spine

• Obstruction of the inferior vena cava by the gravid uterus (especially when supine) distends the epidural venous plexus
• This decreases the volume of the epidural space and increases epidural pressure (positive rather than the usual negative)
• Increases the risk of intravascular injection during epidural anesthesia

Summary Table

Fetal Parts - Appearance on Ultrasound (USG)

Developmental Landmarks by Gestational Age (Transabdominal USG)

Early Pregnancy Structures

Gestational Sac (5-6 weeks)

• Appears as a hypoechogenic (anechoic/dark) area = the chorionic vesicle
• Surrounded by a bright echogenic rim = the trophoblast
• A double echogenic ring is the hallmark of a normal intrauterine pregnancy:
  • Inner ring = decidua capsularis + chorion laeve
  • Outer ring = decidua vera
• Measured inside the hyperechoic rim (anechoic space only); if round - one diameter; if ovoid - average of three diameters

Yolk Sac (from ~5 weeks TVS, ~6-7 weeks TAS)

• Small, round, bright echogenic ring within the gestational sac
• Its presence confirms an intrauterine gestation

Fetal Biometry: The Four Key Measurements

1. Crown-Rump Length (CRL) - 6 to 12 weeks

• The longest sagittal diameter of the fetus, from top of skull to base of rump
• Excludes limb buds and the yolk sac
• Fetus appears as a small elongated echogenic structure within the anechoic amniotic fluid
• Most accurate between 7-9 weeks (TVS) or 9-11 weeks (TAS)
• Formula: Gestational age (weeks) = (CRL [mm] + 65) / 10
• Accuracy: ±3-5 days

2. Biparietal Diameter (BPD) - from 12-13 weeks

• Cross-sectional view of the fetal skull at the level of the thalami
• The skull appears as a bright oval echogenic ring
• Measured outer table to inner table (only outer-to-inner measurement in all of sonography - because the posterior calvarium causes artifact that distorts outer-to-outer measurements)
• Reference plane cuts through: falx cerebri, thalamus, cavum septi pellucidi, and medial cerebral artery
• Head should be oval-shaped at this plane
• Normal cephalic index (BPD:OFD) = 78.3% ± 8%; below = dolichocephalic; above = brachycephalic
• Accuracy: ±1.5 weeks (14-20 wks); ±3-4 weeks (third trimester)

3. Head Circumference (HC)

• Measured in the same BPD plane
• HC = 1.57 × (BPD + OFD)
• More reliable than BPD when skull shape is abnormal

4. Abdominal Circumference (AC)

• Cross-section of the fetal abdomen at the level of the portal vein junction
• Structures visible in this plane:
  • Stomach - seen as a hypoechoic (dark) bubble on the LEFT side of the fetus
  • Umbilical vein (UV) - echogenic curved structure joining the left portal vein
  • Spine (Sp) - echogenic shadowing structure posteriorly
  • Aorta (Ao) and Vena Cava (VC) - visible as small circular structures near the spine
• AC = 1.57 × (transverse diameter + anteroposterior diameter)

5. Femur Length (FL) - from ~14 weeks

• The femoral diaphysis appears as a bright echogenic line (the calcified shaft)
• Measured along the central shaft only - NOT including the epiphysis
• The beam should be as perpendicular to the shaft as possible
• Distal femoral epiphyseal ossification center (echogenic dot at the distal end) appears at 33 weeks or more - a maturity marker
• FL/BPD ratio is constant after 22 weeks = 79% ± 8%

Head and Brain Structures

Cranial Vault

• Appears as a bright echogenic oval ring in cross-section
• Falx cerebri: a midline echogenic line dividing the two hemispheres

Structures Assessed at the BPD Plane

• Thalami - paired symmetric hypoechoic structures at the center
• Cavum septi pellucidi - a small fluid-filled (anechoic) box-like space between the two leaflets of the septum pellucidum; present between 18-37 weeks; its absence suggests holoprosencephaly or agenesis of the corpus callosum
• Lateral ventricles - measured at the atrium; normal width <10 mm (anechoic fluid-filled spaces)

Cerebellum / Posterior Fossa

• Viewed on a plane slightly inferior and posterior to the BPD plane
• Cerebellum: appears as a "dumbbell" or "butterfly" shape - two echogenic cerebellar hemispheres connected by the hypoechoic vermis
• Cisterna magna: anechoic space behind the cerebellum (normal 2-10 mm)
• Nuchal translucency (NT): measured 11-14 weeks - the anechoic translucent space at the back of the fetal neck; thickness >3 mm associated with Down syndrome

Face

• Orbits: two symmetric anechoic circles (the lens appears as a bright echogenic dot)
• Facial profile (sagittal view): nasal bone, forehead, lips, chin all assessable
• Upper lip continuity: assessed to exclude cleft lip (gap in echogenic lip line)

Heart (18-22 weeks)

Four-Chamber View

• Both atria and both ventricles appear as four anechoic (dark) chambers separated by echogenic walls and septa
• Right side (RV, RA) is normally slightly larger than left
• Ventricular septum: echogenic line between ventricles (dropout artifact can mimic VSD - assess with beam perpendicular to septum)
• Foramen ovale flap: visible in the left atrium, normally opens toward LA
• Tricuspid valve inserts slightly more apically (toward apex) than the mitral valve - an important landmark
• Moderator band: characterizes the right ventricular apex (echogenic trabecular band)
• Pulmonary veins drain into the left atrium (visible as small anechoic channels)
• Descending aorta (DA): visible as a small anechoic circle posterior to the heart near the spine

Outflow Tracts

• LVOT (left ventricular outflow tract) and RVOT are also assessed in the standard scan

Thorax / Lungs

• Lung parenchyma: appears as homogeneously echogenic tissue on both sides of the heart
• Lungs should be symmetric; hyperechoic lung mass = may indicate CCAM or sequestration; anechoic pleural fluid = effusion

Abdomen

Stomach

• Appears as a well-defined anechoic (black) cystic bubble in the LEFT upper abdomen
• Absence of stomach bubble raises concern for esophageal atresia or CNS abnormality

Liver and Intestines

• Liver: the largest abdominal organ; moderately echogenic (mid-gray)
• Small bowel loops: mildly echogenic, not as bright as bone; colon is more prominent in late pregnancy
• Umbilical cord insertion into the anterior abdominal wall: confirms normal abdominal wall closure (absence/defect = omphalocele or gastroschisis)

Kidneys

• Located bilaterally flanking the spine
• Renal cortex: slightly less echogenic than adjacent liver/spleen
• Renal pelvis: small anechoic center (fluid); dilated >7 mm = pyelectasis, may indicate obstruction or Down syndrome
• Bladder: oval anechoic structure in the pelvis; should fill and empty cyclically; two umbilical arteries run alongside it (visible with colour Doppler)

Spine

• Longitudinal view: two parallel echogenic lines (posterior elements), converging at the sacrum
• Transverse view: three echogenic dots in a triangle (vertebral body + two posterior elements)
• Normal skin covering overlying the spine should be visible and intact
• Open neural tube defects (spina bifida) show a gap with soft tissue protrusion; associated "banana sign" (cerebellum pulled down, looking like a banana) and "lemon sign" (frontal bossing of skull)

Extremities

• Long bones (humerus, radius, ulna, femur, tibia, fibula): appear as bright echogenic lines with acoustic shadowing (dark shadow behind them)
• All four extremities must be visualized; upper and lower long bones assessed
• Hands and feet: fingers and toes visible; posture and movement assessed
• Shortening, bowing, or fractures suggest skeletal dysplasia

Genitalia

• Male: echogenic scrotum and penis visible, testes may be seen in the scrotum after 28-32 weeks
• Female: three echogenic lines (labial folds) visible in the perineum

Placenta and Amniotic Fluid (Assessed Alongside Fetal Parts)

• Placenta: uniformly granular echogenic structure; classified by Grannum grading (Grade 0-III for maturity)
• Umbilical cord: contains 2 arteries + 1 vein; appears as a 3-vessel cord on transverse section
• Amniotic fluid: anechoic (black); assessed by amniotic fluid index (AFI) or single deepest pocket

Accuracy of USG Dating