Assessment of the third trimester
asymmetric intrauterine growth restriction
most common type of growth abnormality demonstrating normal cranial growth and a decrease in abdominal growth.
objective means for assessing fetal well-being.
systolic pressure ≥140 mm Hg or a diastolic pressure ≥90 mm Hg.
amniotic fluid below the normal range for gestational age.
amniotic fluid above the normal range for gestational age.
gestation greater than 42 weeks.
a condition in which accelerated fetal growth results in an infant with a birth weight greater than 4000 g; associated with birth asphyxia and trauma.
symmetric intrauterine growth restriction
fetal growth abnormality resulting in a proportionally small fetus.
fatty material found on the fetal skin and amniotic fluid late in pregnancy.
Third-trimester measurements
• Abdominal circumference (AC).
• Head circumference-to-abdominal circumference ratio (HC/AC).
• During the early third trimester, the head circumference is slightly larger than the circumference of the abdomen.
• During the late third trimester, with the increase of fetal body fat, the abdominal circumference is typically equal to or slightly larger than the head circumference.
Fetal growth
• Interval fetal growth can be determined with ultrasound examinations a minimum of 3 weeks apart.
• In the last 3 months of pregnancy, the fetus will grow an additional 4 inches in length and gain an additional 2000-2800 g in weight at 100-200 g per week.
• Distal femoral epiphysis (DFE) is visualized around 32 gestational weeks.
• Proximal tibial epiphysis (PTE) is visualized around 35 gestational weeks.
Decrease in fetal growth
Intrauterine growth restriction
• Results from insufficient fetal nutrition.
• Defined as a fetal weight at or below the 10th percentile for gestational age.
• No single reliable criterion is available to diagnose intrauterine growth restriction.
• Associated with maternal hypertension.
• Evaluation of the amniotic fluid volume, estimated fetal weight, and maternal blood pressure results in the most accurate diagnosis.
• The liver is one of the most severely affected fetal organs.
• Decrease in liver size results in a decrease in abdominal circumference.
Intrauterine Growth Restriction
TYPE | ETIOLOGY | CLINICAL FINDINGS | SONOGRAPHIC FINDINGS | DIFFERENTIAL CONSIDERATIONS |
Asymmetric | Placental insufficiency (most common)Chromosomal abnormalityInfectionMaternal Risk Factors Hypertension (most common) Poor nutrition Alcohol and drug abuse | Small for datesLow maternal weight gainHypertension | Lack of fetal growth on serial sonogramsDecrease in abdominal circumferenceNormal head circumference and femur lengthDecrease in amniotic fluid volumeIncrease in HC/AC ratioPlacentomalaciaGrade 3 placentaUmbilical ArterySystolic–diastolic ratio of umbilical artery >3.0 after 30 wksAbsence or reversal of diastolic flow is considered criticalUmbilical VeinDecrease in flow volume | Normal small fetusSkeletal dysplasia |
Symmetric | Result of embryologic insult | Small for dates | Symmetrically small head and abdomen circumferenceOligohydramnios | Incorrect menstrual datesNormal small fetusSkeletal dysplasia |
Increase in fetal growth
Macrosomia
• Fetal weight above 4000 g or above the 90th percentile for gestational age.
• Fetuses of diabetic mothers are likely to display organomegaly, whereas fetuses of nondiabetic mothers will demonstrate normal growth.
• Fetuses of diabetic mothers demonstrate a higher mortality rate.
CONDITION | ETIOLOGY | CLINICAL FINDINGS | SONOGRAPHIC FINDINGS | DIFFERENTIAL CONSIDERATIONS |
Macrosomia | Maternal diabetes mellitusMaternal obesityPostterm pregnancy | Large for dates | Large abdominal circumferenceDecreased HC/AC ratioEstimated fetal weight >4000 gPolyhydramniosPlacentomegaly | Normal large fetusSuboptimal fetal measurements |
Amniotic fluid
• Normal volume of amniotic fluid varies with gestational age.
• Early in gestation, the major source of amniotic fluid is the amniotic membrane.
• As the embryo and placenta develop, fluid is produced by the placenta and fetus.
• After 16 gestational weeks, the fetus is the major producer of amniotic fluid.
Amniotic fluid volume
• Normal volume of amniotic fluid increases progressively until about 33 gestational weeks.
• During the late second and early third trimester, the amniotic fluid volume appears to surround the fetus.
• By the late third trimester, the amniotic fluid displays as isolated fluid pockets.
• Regulated by the production of fluid, swallowing of fluid (removal), and fluid exchange within the lungs, membranes, and cord.
• Normal lung development depends on the exchange of amniotic fluid within the lungs.
• Oligohydramnios increases risk of fetal death and neonatal morbidity.
Measuring amniotic fluid volume
• Transducer must remain perpendicular to the maternal coronal plane and parallel to the maternal sagittal plane.
• Fluid pocket must be free of umbilical cord or any fetal part.
Methods of Assessing Amniotic Fluid Volume
METHOD | DESCRIPTION | NORMAL SONOGRAPHIC FINDINGS | ABNORMAL SONOGRAPHIC FINDINGS |
Amniotic fluid index (AFI) | Determined by dividing the uterus into four equal partsMeasure deepest unobstructed pocket in each quadrantAFI is equal to the sum of all four quadrants | AFI >5 cm and <24 cm | AFI ≤5 cm or >24 cm |
Single deepest pocket | Maximum vertical depth of any amniotic fluid pocket | Largest pocket >2 cm and <8 cm | Largest pocket <1 cm or >8 cm |
Subjective assessment | Observing the amount of amniotic fluid during real-time examinationExperience increases accuracy | Amount of amniotic fluid appears within normal limits for gestation | Amniotic fluid appears greater or less than expected for the gestational age |
Abnormal Amniotic Fluid Volume
ABNORMALITY | ETIOLOGY | SONOGRAPHIC FINDINGS | DIFFERENTIAL CONSIDERATIONS |
Oligohydramnios | FetalGenitourinary tract abnormalityIntrauterine growth restrictionMaternalPoor nutritionPlacenta insufficiencyPremature rupture of membranes | AFI below 5 cmBelow the 5th percentile for gestational ageLargest single pocket below 1 cmPoor fetal–fluid interfaceVolume <300-500 mL | Lower limits of normalPremature rupture of membranes |
Polyhydramnios | Fetal Anomalies
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