For purposes of the US Zika Pregnancy Registry (the Registry), any of the following laboratory findings are considered evidence* of possible Zika virus infection:
Positive results from laboratory testing for Zika virus particles (RNA NAT [nucleic acid testing]) positive (+) in any clinical specimen (such as whole blood, serum, urine, cerebrospinal fluid, amniotic fluid, or tissue samples)
Evidence of an immune response to a recent virus that is likely to be Zika virus
Serum Zika IgM positive (+), equivocal or inconclusive (-) AND Zika plaque reduction neutralization test (PRNT) titer ? 10 or
Serum Zika IgM positive (+) AND Zika PRNT not done (e.g. per local/territorial health department protocol)
Evidence on histopathologic evaluation of the placenta, umbilical cord or other fetal tissue with Zika virus immunohistochemical staining on fixed tissue or Zika virus RNA NAT testing on fixed and frozen tissue
*Reviewed and updated on an ongoing basis
Exposure Time Periods
Periconceptional exposure is defined as maternal Zika virus infection during the 8 weeks before conception (6 weeks before the first day of the last menstrual period).
Prenatal exposure is defined as fetal exposure to a maternal Zika virus infection during pregnancy but before delivery.
Perinatal exposure is defined as fetal exposure to maternal Zika virus infection at or around the time of delivery, when a woman is infected with Zika virus within approximately 2 weeks of delivery.
Gestational Age and Trimesters
The estimated date of delivery (EDD) is a key metric for the US Zika Pregnancy Registry. For a given pregnancy, there should be one EDD established early in pregnancy, based either on the first day of the last menstrual period (LMP) or by ultrasound measurements. EDD estimates are generally determined early in pregnancy and may be confirmed by ultrasound. Ultrasounds early in pregnancy are more accurate to confirm EDD than those later in pregnancy. When questions arise about the most accurate EDD, CDC will work with the health department and if requested, the healthcare provider, to determine the most accurate EDD for the USZPR. It is important to include how EDD was established because this information assists in determining exposure timing and gestational age.
The first trimester begins at 2 weeks after the last menstrual period (2 weeks gestation) and extends for 13 weeks plus 6 days.
The second trimester begins at 14 weeks gestation and extends until 27 weeks and 6 days.
The third trimester begins at 28 weeks gestation and extends until delivery.
Conventions for gestational notation are based on the 7-day week, e.g. gestational age of 25 plus 4 means 25 weeks and 4 days.
Microcephaly
Zika virus infection during pregnancy is a cause of microcephaly and other severe fetal brain defects.
Microcephaly is diagnosed when an infant’s head is smaller than expected as compared to infants of the same age (or gestational age) and sex.
Microcephaly can be determined by measuring head circumference (HC) after birth.
Although head circumference measurements may be influenced by molding and other factors related to delivery, the measurements should be taken on the first day of life because commonly-used birth head circumference reference charts by age and sex are based on measurements taken before 24 hours of age. The most important factor is that the head circumference is carefully measured and documented. If measurement within the first 24 hours of life is not done, the head circumference should be measured as soon as possible after birth.
To accurately measure HC use a measuring tape that cannot be stretched. Securely wrap the tape around the widest possible circumference of the head, which is usually one to two finger widths above the eyebrow on the forehead, and then across the most prominent part of the back of the head. This measurement should be repeated 3 times in a row, and the largest measurement to the nearest 0.1 centimeter should be selected.
