Prenatal Screening Tests

Routine Lab testing in prenatal care:

1st Trimester (1 - 14 weeks):

• • CBC: anemia = Hb <10 g/dL (normal 10 - 12).

    • ▼ MCV and other RBC indices low, and ▲ RDW. Iron deficiency most common cause. Give iron. Test thalassemia if anemia does not improve.

    • ▼ Hb, ▲ MCV, ▲ RDW: Give folate

    • WBC >16,000/mm3 is abnormal

    • Thrombocytopenia (<150,000/mm3): Correlate clinically with ITP or HELLP syndrome.

  • Blood type, Rh and Ab screen (direct and indirect Coombs for AAT)

    • Rh negative mothers may become sensitized (anti-D Ab) → risk of erythrobalstosis fetalis in the next pregnancy.

      • Give RhoGAM at 28 weeks after first screening for absence of anti-D abs (DAT).

    • Indirect Coombs test for atypical antibody (AAT) detects atypical RBC Ab's.

  • Genitourinary screening

    • Cervical PAP smear: Detects cervical dysplasia or malignancy. Refer to Gyn for management.

    • UA and urine culture: Screen underlying UTI. Screen for asymptomatic bacteriuria (ASB)

      • Always treat ASB in pregnancy to prevent pyelonephritis (30% risk if untreated). Rx: Nitrofurantoin, cephalosporins, amoxicillin

  • Immunization Status

    • Rubella ab. if negative Rubella IgG, it means mother is at increased risk of primary rubella infection.

    • Do not give rubella immunization in pregnancy. Immunize seronegative patient after delivery.

    • HbsAg. If positive indicates risk for vertical transmission of HBV.

    • Order HBeAg. If positive, indicates highly infectious state.

  • Infectious syphilis: VDRL or RPR

    • Confirm positive VDRL/RPR with MHATP or FTA-ABs. If positive treat with PCN IM. PCN allergic, desensitize and then treat with PCN.

  • HIV (ELISA). Confirm ELISA with Western blot test (presence of HIV core and envelope antigen).

    • Always get consent for HIV testing. Not needed now.

    • All babies born to HIV +ve mothers will have HIV ab +ve due to passive transport of maternal Abs. +ve Abs in infants do not necessarily mean infection in infant.

    • Antiretrovirals are not C/I in pregnancy.

  • Cervical culture for Gonorrhea, Chlamydia and Trichomonas.

    • Gram stain, G & C, Trichomonas vaginalis (can cause premature labor).

    • Tx: Azithromycin PO and Ceftriaxone IM. Alteranative, Amoxicillin PO.

    • Bacterial vaginitis/vagniosis: Metronidazole, PO or clindamycin PO

• Optional tests:

  • • PPD. Test for exposure to TB in high risk mothers. +ve test is induration, not erythema.

    • Negative PPD: no further follow-up is needed.

    • Positive PPD: CXR to rule out active disease.

      • Treat +ve PPD /-ve CXR: INH and B6 for 9 months.

      • +ve PPD / +ve CXR / +ve sputum: Triple therapy anti-TB. Avoid streptomycin in pregnancy because of the risk of ototoxicity in the fetus.

    • Trisomy testing (ß-HCG, PAPP-A - pregnancy associated plasma protein A; fetal nuchal translucency at 10 wks), offered to high-risk pregnancies (females >35 at delivery, women with a history of prior trisomy 21)

      • +ve screening test is confirmed with CVS in 1st trimester.

2nd Trimester (14 - 26 weeks):

• • 1 hr 50 gm OGTT given between 24 – 28 wks. No fasting needed.

    • Abnormal result: BS >140 mg/dL. Do 3 h, OGTT. Needs fasting ON.

  • Anemia

    • CBC measured between 24 - 28 weeks. Hb <10 g/dL = anemia. Iron deficiency is the most common cause, even if not present in 1st trimester.

    • Give Iron supplementation.

  • Atypical antibodies:

    • Indirect Coombs test performed in Rh-negative women to look for atypical anti-D-Ab before giving RhoGAM.

  • RhoGAM is not indicated in Rh negative women who have developed anti-D antibodies.

  • GBS screening (vaginal and rectal culture) at 35 – 37 weeks.

    • +ve GBS culture is a high risk for sepsis in newborn.

    • Presumptive treatment based on certain risk factors for neonatal infections, including preterm labor, preterm premature rupture of membrances (P PROM), prolonged rupture of membranes, intrapartum fever, and a previous neonate with GBS infection.

    • Treat with intrapartum IV ABx. PCN G IV, clindamycin IV or eyrthromycin in PCN allergic patients.

• Optional tests:

  • • Quadruple marker screen (MSFAP, ß-HCG, estriol, add dimeric inhibin-A in high risk women)

      • 16 - 20 weeks gestation.

    • MS-AFP is measured in multiple of the median (MoM). It increases with gestational age.

      • > 2.2 MoM is considered elevated.

      • < 2.5 MoM is considered normal.

    • Inhibin A is made by the placenta during pregancy and normally remains constant during 15 - 18 week of pregnancy. The level of inhibin A is increased in the blood of mothers of fetuses with Down syndrome.

    • ▲ MS-AFP: NTD, ventral wall defect, twin pregnancy, placental bleeding, sacrococcygeal teratoma.

    • Abnormal MS-AFP, perform US to confirm dating of pregnancy. If dating error, repeat MS-AFP. Normal repeat MS-AFP is reassuring. If repeat MS-AFP is elevated, do US.

    • If MS-AFP > 7 MoM and family history of NTDs: do TVUS

      • If ▲ MS-AFP: do amniocentesis for amniotic fluid AF-AFP level and acetylcholinesterase activity. Elevated levels of amniotic fluid-acetycholinesterase activtiy is specific to open NTD.

      • For ▼ MS-AFP: amniocentesis for karyotyping.

    • ▼ MS-AFP: Trisomy 21, Trisomy 18

    • Trisomy 21 (Down syndrome): ▼ MS-AFP, ▼ Estriol, ▲ Beta-hCG, ▲ Inhibin-A

    • Trisomy 18 (Edward syndrome): ▼ MS-AFP, ▼ Estriol, ▼ Beta-hCG.

Parameters routinely measured on subsequent visits:

BP, wt., ut. fundal ht., fht, fetal presentation and activity, urine glucose and protein.

Anemia Hb <10 g/dL.

• Scuba diving in pregnancy increases the risk of decompression injury and gas emboli in fetus

• In hypothyroidism pregnancy check TSH every trimester.