The practice of medicine continues to evolve, and individual circumstances will vary. This publication reflects information available at the time of its submission for publication and is neither designed nor intended to establish an exclusive standard of perinatal care. This publication is not expected to reflect the opinions of all members of the Society for Maternal-Fetal Medicine.
Microcephaly is a condition in which the size of the head is smaller than expected for age. This condition in fetuses and infants has been associated with the recent outbreak of Zika virus. Due to this association, the Centers for Disease Control and Prevention (CDC), American Congress of Obstetricians and Gynecologists (ACOG), and Society for Maternal-Fetal Medicine (SMFM) have suggested prenatal ultrasound evaluation for fetal microcephaly in pregnant women who have been infected or potentially exposed. However, the diagnosis of microcephaly by prenatal sonography is not always straightforward. Given the complexity of prenatal diagnosis of microcephaly, the purpose of this document is to review the ultrasound criteria for the diagnosis following exposure to the Zika virus.
Various national and international agencies have recommended prenatal ultrasound for evaluation for fetal microcephaly in women who have travelled to any of the high-risk areas for Zika exposure during pregnancy. At present, however, there are limited data available regarding criteria for diagnosis of fetal microcephaly in the setting of Zika infection or exposure. In addition, the natural history of fetal microcephaly associated with Zika virus is unknown; although recent reports describe cases of microcephaly after maternal infection. In most cases, it is difficult to differentiate between constitutionally small head size vs pathologic microcephaly, and available data regarding prenatal diagnosis of microcephaly are based on small numbers of cases of varying etiologies.
In cases in which the fetal head circumference (HC) measures >2SD below the mean, we recommend that a detailed neurosonographic examination be performed, as some fetuses with HC >2SD below the mean due to in utero infection will have findings such as periventricular and intraparenchymal echogenic foci, ventriculomegaly, cerebellar hypoplasia, microcephaly, and cortical abnormalities. In addition, assessment of the profile can be helpful as the forehead is often sloping in pathologic microcephaly, and demonstration of this finding should increase the index of suspicion. We recommend that isolated fetal microcephaly should be defined as fetal HC ≥3SD below the mean for gestational age ( Table ), and the diagnosis of pathologic microcephaly is considered certain when the fetal HC is ≥5SD. If the HC by prenatal ultrasound is >2SD below the mean, a careful evaluation of the fetal intracranial anatomy is indicated. If the intracranial anatomy is normal, we recommend follow-up ultrasound in 3-4 weeks.
Gestational age, wk | Mean, mm | Head circumference, mm: SD below mean | ||||
---|---|---|---|---|---|---|
–1 | –2 | –3 | –4 | –5 | ||
20 | 175 | 160 | 145 | 131 | 116 | 101 |
21 | 187 | 172 | 157 | 143 | 128 | 113 |
22 | 198 | 184 | 169 | 154 | 140 | 125 |
23 | 210 | 195 | 180 | 166 | 151 | 136 |
24 | 221 | 206 | 191 | 177 | 162 | 147 |
25 | 232 | 217 | 202 | 188 | 173 | 158 |
26 | 242 | 227 | 213 | 198 | 183 | 169 |
27 | 252 | 238 | 223 | 208 | 194 | 179 |
28 | 262 | 247 | 233 | 218 | 203 | 189 |
29 | 271 | 257 | 242 | 227 | 213 | 198 |
30 | 281 | 266 | 251 | 236 | 222 | 207 |
31 | 289 | 274 | 260 | 245 | 230 | 216 |
32 | 297 | 283 | 268 | 253 | 239 | 224 |
33 | 305 | 290 | 276 | 261 | 246 | 232 |
34 | 312 | 297 | 283 | 268 | 253 | 239 |
35 | 319 | 304 | 289 | 275 | 260 | 245 |
36 | 325 | 310 | 295 | 281 | 266 | 251 |
37 | 330 | 316 | 301 | 286 | 272 | 257 |
38 | 335 | 320 | 306 | 291 | 276 | 262 |
39 | 339 | 325 | 310 | 295 | 281 | 266 |
40 | 343 | 328 | 314 | 299 | 284 | 270 |
41 | 346 | 331 | 316 | 302 | 287 | 272 |
42 | 348 | 333 | 319 | 304 | 289 | 275 |