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2017; 63(1):4-6

ing the exact date of the last menstrual period or the

misinterpretation of genital bleeding related to egg im-

plantation as menstrual bleeding. In addition, the conven-

tion of a 14-day interval between menstruation and ovu-

lation may render gestational age calculations inaccurate,

especially for women with irregular menstrual cycles.

(

)

Finally, studies have consistently shown that the use

of the date of the last menstrual period for the purpose of

estimating gestational age is related to a greater frequency

of a later due date compared to that which is verified when

this estimate is obtained from the early ultrasound (10

to 12% versus 4%, respectively).

3,6

(

)

Early detection of a multiple gestation

It is known that multiple pregnancies are associated with

increased perinatal morbidity and mortality compared

to single fetus pregnancies. Therefore, early identification

of multiple pregnancies and determination of the type of

placenta play an important role in the risk stratification

and monitoring of twin pregnancies, contributing sig-

nificantly to a better prognosis. Observational studies

designed to analyze the accuracy of ultrasonography per-

formed during the first trimester in multiple pregnancies

to predict chorionicity consistently found high values of

sensitivity, specificity, positive and negative predictive

values compared to those observed from ultrasonographies

performed in the second trimester.

7,8

(

) Thus, ultrasound,

especially when performed by the end of week 14, is a

reliable tool for determining the number of chorions in

a twin pregnancy.

Evaluation of fetal morphology

First trimester ultrasound, performed between week 11

and week 14 of pregnancy, aims to track chromosomal

abnormalities, genetic syndromes and other fetal mal-

formations. This imaging method is well-established

for the screening of aneuploidies. In 2008, Kagan et al.

showed that the increase in cutaneous thickness pres-

ent in individuals with Down syndrome could be seen

in the first trimester (from 11 to 13 weeks plus 6 days).

They also verified that increased nuchal translucency

(thickness above the 95

percentile for gestational age),

when associated with maternal age and biochemical

tests such as maternal serum levels of beta-hCG free

fraction and pregnancy-associated plasma protein

(PAPP-A) provided a detection rate of 90% of cases of

trisomy 21 with a false-positive rate of 3%.

(

) Even in

the absence of chromosomal abnormality, fetuses ex-

hibiting increased nuchal translucency have an increased

risk of intrauterine death and structural abnormalities,

especially cardiac.

(

)

Some abnormalities like anencephaly are almost al-

ways detected. However, others such as myelomeningocele

or microcephaly may be difficult or impossible to iden-

tify. Numerous studies analyzing the performance of first

trimester ultrasonography to detect fetal abnormalities

found that the findings were either incidental during

screening for aneuploidy or were detected after careful

TABLE 1

Risk factors that may indicate referral to

high-risk prenatal care.

Factors related to previous conditions

Heart diseases, severe lung diseases (including bronchial asthma),

severe kidney diseases (such as chronic renal failure and transplant

patients), endocrine diseases (particularly diabetes mellitus,

hypothyroidism, and hyperthyroidism), hematological disorders

(including sickle cell disease and thalassemia), chronic hypertension

and/or patients treated with antihypertensive medication (PA >

140/90 mmHg before gestational age of 20 weeks), neurological

diseases (such as epilepsy), psychiatric disorders requiring monitoring

(psychosis, severe depression, etc.), autoimmune diseases (systemic

lupus erythematosus, other collagenoses), maternal genetic diseases,

history of deep venous thrombosis or pulmonary embolism,

gynecological disorders (uterine malformation, myomatosis, adnexal

tumors, and others), patients with infectious diseases such as

hepatitis, toxoplasmosis, HIV infection, tertiary syphilis (USG with

fetal malformation), and other STDs (condyloma), Hansen’s disease,

tuberculosis, licit or illicit drug addiction, any clinical pathology

that requires specialized monitoring.

Factors related to previous reproductive history

Intrauterine or perinatal death in previous gestation, especially if the

cause is unknown; previous history of hypertensive gestational disease

with poor obstetric and/or perinatal outcome (premature termination

of pregnancy, intrauterine fetal death, Hellp syndrome, eclampsia,

maternal ICU admission); repeated abortion; infertility.

Factors related to the current pregnancy

Restriction of intrauterine growth; polyhydramnios or oligohydramnios;

twin pregnancy; fetal malformations or fetal arrhythmia; hypertensive

disorders of gestation (pre-existing chronic hypertension, gestational

or transient hypertension); recurrent urinary tract infection or two

or more episodes of pyelonephritis; severe or unresponsive anemia

after 30-60 days of treatment with ferrous sulfate; patients with

infectious diseases such as hepatitis, toxoplasmosis, HIV infection,

tertiary syphilis (USG with fetal malformation), and other STDs

(condyloma); infections such as rubella and cytomegalovirus acquired

in the current gestation; laboratory evidence of proteinuria; gestational

diabetes mellitus; severe maternal malnutrition; morbid obesity or

low weight.

Adapted from: Ministério da Saúde, 2012.