Fetal Assessment and Prenatal Diagnosis

Routine Prenatal Care

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  • Ideally starts before conception.
  • Minimum interval for prenatal visits for women with uncomplicated pregnancies: 4–5 wk until 28 wk of gestation, 2–3 wk from 28–36 wk of gestation, and then weekly until delivery.
  • Prenatal visit: Initial visit should include comprehensive history, physical examination, and extensive patient education. Subsequent visits assess maternal and fetal well being, maternal weight gain with review of nutritional intake, fundal height, BP, and urine screening for asymptomatic UTI and proteinuria.

Routine Tests Performed at the Initial Prenatal Visit

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Test

Comments

Rhesus type, antibody screen

Screen for antibodies that may result in hemolytic disease of the newborn. Rh- women should receive anti(D)-immune globulin prophylaxis (current recommendations are at 28 wk and at delivery, as well as with any invasive procedure in which maternal–fetal circulation can mix such as amniocentesis or CVS)

CBC

Screen for anemia, hemoglobinopathies

Rubella immunity screen

Nonimmune patients should be immunized postpartum (not during pregnancy!)

RPR

Treatment of mother and infant may be indicated for positive serologies (see chapter 38)

HepBsAg

Immunoprophylaxis at delivery for positive serologies

UA and urine culture

Treatment of asymptomatic bacteria indicated in pregnancy due to increased risk of perinatal morbidity, preterm labor

Cervical cytology

If not done within 6 months of pregnancy

Chlamydia and gonorrhea endocervical specimen

Repeat testing if initial results are positive, in women <25 yr old, and high-risk women

HIV

ACOG recommends universal screening with an “opt-out” strategy

Routine Screening Performed during Second and Third Trimesters

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When To Screen

Comments

OGTT

24–28 wk

Should be done in first trimester in women with risk factors (obesity, prior history of GDM, prior macrosomic infant)

Repeat STD screening (RPR, HepBsAg, Chlamydia and gonorrhea)

Third trimester

Only women at continued risk and those who acquired a new risk factor during pregnancy

CBC, antibody screening

Early third trimester

Vaginal and rectal swab for GBS

35–37 wk

Also in the event of PPROM as outlined below

Screening Tests for Selected Populations and Controversial Screening Tests

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Disease

Test

Comments

Bacterial vaginosis

Not recommended for routine screening

HCV

HCV antibody screening

No recommended screening date; done at first prenatal visit

Screen only patients at high risk for disease (IVDU, blood products, liver disease)

HSV

Not recommended for routine screening

Thyroid dysfunction

TSH, free T4

Controversial; ACOG and Endocrine Society recommend testing only for women who are symptomatic, have a personal or family history of disease, or are otherwise at high-risk

Because of the possible adverse impact on neurologic development of undetected hypothyroidism, others recommend universal screening

Tuberculosis

PPD placement

Pregnant women should be tested in accordance with guidelines established for nonpregnant patients.

Toxoplasmosis

Toxoplasmosis IgG and IgM serologies

Routine screening of pregnant women is controversial and often not performed in the US

Varicella

Varicella serologies

Test all pregnant women; provide counseling for seronegative women and postpartum varicella vaccine

Screening Us

  • First trimester: Routine screening of an unselected population allows for better estimation of GA, leading to reduced frequency of labor induction for postterm labor and use of tocolysis for suspected preterm labor; detection of multifetal pregnancies, and anatomic “markers” suggestive of increased risk for chromosomal abnormalities or fetal malformations (see below).
  • Second trimester (anatomy scan): Allows detection of 50%–60% of all congenital anomalies with experienced users. Ideally performed between 18–20 wk gestation.
  • Third trimester: Current data do not support the routine use of US in the third trimester.

Aneuploidy Screening

Routinely offered to all pregnant women (see screening for fetal chromosomal anomalies below).

Assessment of Gestational Age

  • Initial assessment of GA is made based on a careful menstrual history.
  • The estimated date of confinement (EDC) or estimated date of delivery (EDD) is calculated by adding 7 d to the first day of the LMP and then counting back 3 mo.
  • Note that conceptual age is not equivalent to GA. For an idealized 28-d menstrual cycle, ovulation occurs approximately 2 wk after the LMP.
  • This leads to frequent confusion in terminology both before and after delivery.

Commonly Used Terms

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Term

Definition

Units of Time

Gestational age

Time elapsed between first day of the LMP, up to the day of delivery

Completed weeks

Chronological age

Time elapsed since birth

Days, weeks, months, years

Postmenstrual age

GA + chronological age

Weeks

Corrected age

Chronological age reduced by the number of weeks born before 40 wk of gestation

Weeks, months

eFigure 29-1.

Summary of age terminology in the perinatal period.

  • Pelvic examination by an experienced provider is accurate in determining GA within 1–2 wk until the second trimester. However, this method is seldom used in isolation to estimate GA.
  • From 16–18 wk of gestation until 36 wk of gestation, the fundal height in centimeters is roughly equal to GA in weeks. However, this finding is more useful to help monitor fetal growth than it is to estimate GA.
  • The most accurate measurement of GA currently available is obtained with US examination.
    • Various nomograms of fetal growth and formulas have been developed to estimate GA and are integrated into most modern obstetric US equipment.
    • Estimates are most accurate when multiple parameters are used, and nomograms are based on data obtained from fetuses of the same ethnic or racial background living at similar altitudes.
    • The most accurate measurement during the first trimester is the crown–rump length (variation of 3–5 d).
    • From 14–26 wk, the biparietal diameter (BPD) becomes the most accurate parameter (variation of 7–10 d).
    • The femur length (FL) can also be used during the second trimester. It correlates well with the BPD and has a variation of 7–10 d.
    • The abdominal circumference (AC) is the most variable parameter commonly measured.
    • Formulas integrating measurements of BPD, HC, AC, and FL are more useful than single measurements during the third trimester, although these estimates are still less accurate than earlier dating. Caution should be used in using composite GA assignment (averaging of all biometric parameters) with second trimester US because this will reduce the detection of fetal malformation (BPD may be reduced in fetus affected with spina bifida) and chromosomal abnormalities. (Fetuses with Down syndrome have increased incidence of rhizomelic shortening.)

Estimation of Fetal Age with Sonography

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Time Period

Most Accurate Parameter

Variation

First trimester

Crown–rump length

3–5 d

Second trimester

Biparietal diameter

7–10 d

Third trimester

Integration of multiple measurements only

3 wk

Assessment of Fetal Lung Maturity

  • Fetal lung maturity should be assessed before elective delivery at less than 39 wk unless lung maturity can be inferred from any of the following:
    • Fetal heart tones have been documented for at least 20 wk by non-electronic fetoscope or at least 30 wk by Doppler.
    • 36 wk have elapsed since a serum or urine hCG pregnancy test result was reported to be positive.
    • US dating during first or second trimester supports a GA of 39 wk or greater.

Biochemical Tests

  • Lecithin:sphingomyelin ratio (L:S ratio)
    • Both substances are phospholipids “excreted” from fetal lungs into amniotic fluid.
    • Whereas sphingomyelin concentration remains relatively constant throughout pregnancy, lecithin increases from 32–33 wk onward.
    • The risk of RDS is low when the L:S ratio is >2.
  • Phosphatidylglycerol (PG)
    • Increases several weeks after lecithin.
    • Indicative of advanced fetal lung development because it enhances spread of phospholipids in alveoli.
    • Assay for PG is not generally affected by common contaminants in amniotic fluid.
    • Result may be reported qualitatively or quantitatively. The risk of RDS is low when the concentration ≥0.3.
  • Fluorescence polarization (eg, TDx-FLM)
    • Uses polarized light to directly measure the concentration of surfactant in amniotic fluid relative to the concentration of albumin.
    • Elevated ratio of surfactant to albumin (55 mg of surfactant to 1 g of albumin) reflects fetal lung maturity and a low incidence of RDS.
    • Blood and meconium interfere with assay.

Biophysical Tests

  • Foam stability index (FSI)
    • Based on the ability of surfactant to produce “foam.”
    • Ethanol is added to a sample of amniotic fluid, and the mixture is shaken. If surfactant is present in sufficient quantity, foam will form.
    • Serial dilutions are performed to calculate an FSI.
    • RDS is extremely unlikely after a positive test result (FSI ≥47).
  • Lamellar body count
    • Concentration of lamellar bodies in amniotic fluid can be measured directly by light microscopy or indirectly by photospectroscopy.
    • Lamellar bodies are direct evidence of surfactant production by type II pneumocytes.

Evaluation of Fetal Lung Maturity

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Test

Threshold Value to Indicate Maturity

Predictive Value for Maturity When Test Shows Maturity (%)

Predictive Value for Immaturity When Test Shows Immaturity (%)

Relative Cost

Affected by Blood or Meconium?

Can Pooled Vaginal Specimen Be Used?

L:S ratio

>2.0

95–100

33–50

High

Yes

No

PG

If present, indicates maturity

95–100

23–53

High

No

Yes (bacteria may cause false-positive)

FSI

>47–48

95

51

Low

Yes

No

FLM-TDx

>55 mg/g

96–100

47–61

Moderate

Yes

Yes

Lamellar body count

≥50,000

97–98

29–35

Low

Blood only

Unknown

Adapted from ACOG Practice Bulletin No. 97: Fetal Lung Maturity. Obstet Gynecol 2008;112:717.

The Fetal Environment: Maternal Medical Conditions

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Diabetes Mellitus during Pregnancy

  • Definition: Carbohydrate intolerance of variable severity with onset or first recognition during pregnancy.
  • Epidemiology
    • 3%–5% of pregnancies are complicated by DM; of these, 80%–90% represent GDM.
    • Risk factors for the development of GDM: Age >25 yr, obesity, family history of type II DM, ethnic group (Hispanic, African American, Native American, South or East Asian or Pacific Island ancestry), persistent glucosuria, hypertension, dyslipidemia, history of prior pregnancies complicated by GDM, idiopathic macrosomia, congenital malformation, or stillbirth.
  • Pathophysiology
    • In normal pregnancy: Decreased glucose tolerance → increased insulin levels → euglycemia.
    • In both type II DM and GDM: Decreased hepatic and peripheral insulin sensitivity and relatively decreased insulin response.
    • GDM is characterized by a 60% decrease in peripheral insulin sensitivity and an inability of the pancreas to produce adequate insulin in response to glucose load.
    • Several hormones produced by placenta may antagonize effects of insulin: Somatotropin, progesterone.
  • Signs and symptoms: Identical to signs and symptoms of hyperglycemia in nonpregnant patients (fatigue, polyuria, polydipsia, weight loss, polyphagia, blurred vision).
  • Diagnosis
    • Screening routinely performed at 24–28 wk GA with 50-g load, 1-h OGTT. Cutoff: 130–140 mg/dL.
      • 140 mg/dL: Sensitivity, 75%; specificity, 85%–94%
      • 130 mg/dL: Sensitivity, nearly 100%; specificity, 75%–85%
    • Abnormal test should be followed by 75-g or 100-g, 3-h diagnostic OGTT. A diagnosis of GDM is established when two or more of the following criteria are met:
      • Fasting blood glucose >95 mg/dL
      • 1-h blood glucose >180 mg/dL
      • 2-h blood glucose >155 mg/dL
      • 3-h blood glucose >140 mg/dL
    • Women who meet only one of the above criteria should undergo repeat testing during the third trimester.
    • Hemoglobin A1C is not a useful test for the diagnosis of GDM because of the increased rate of erythropoiesis during pregnancy.
  • Treatment
    • There are many proposed regimens for glucose monitoring, including self-monitoring of blood glucose with checks before each meal, 1 or 2 h after each meal, and at bedtime.

Normal Range of Blood Glucose Level: Fasting and Post-Prandial

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Time

Target Glucose Level (mg/dL)

Before meals and snacks

60–95

1 h after meals

130–140

2 h after meals

≤120

2–6 am

60–90

  • Maintaining mean glucose <100 mg/dL during pregnancy reduces perinatal mortality to baseline.
  • Daily urine tests for ketones.
  • Weight management for overweight and obese patients.
  • When pharmacologic therapy is needed to adequately control blood glucose, oral glyburide is most commonly used as the first line therapy in the United States.
  • Screening the fetus: Perform detailed anatomy US at 18–20 wk and fetal echocardiogram at 20–22 wk. Consider serial US monitoring of fetal size and well-being during the third trimester at 4- to 6-week intervals.

Antenatal Testing

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Severity of Disease

When to Start Antenatal Testing

GDM managed with diet alone

36 wk; may consider assessment of fetal movements by mother only

DM requiring insulin therapy

32 wk

DM with maternal complications

Consider starting at <32 wk

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Jan 9, 2019 | Posted by in PEDIATRICS | Comments Off on Fetal Assessment and Prenatal Diagnosis

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