Is Zika suspected of causing birth defects only if contracted during pregnancy?
As you mentioned, there is evidence that mother-child transmission of Zika virus causes congenital anomalies like microencephaly.
Now your question, if a cleared infection before the pregnancy could affect it? In principle, no. Once the virus is cleared, it should not reactivate, unless there is reinfection. Here's a quote from the Zika article from Uptodate.com:  


  Women of childbearing potential — It is uncertain how long women of childbearing potential should wait between Zika virus exposure/infection and conception. Most pregnant women with Zika virus exposure give birth to normal infants [111]. Zika virus usually remains in the blood of an infected person for a few days to a week, and thus far there is no evidence that a fetus conceived after the virus has cleared from the blood would be at risk for Zika infection [94,112]. However, information regarding the persistence of Zika virus following infection is not known. It has been suggested that infection due to other flaviviruses such as West Nile virus may persist years after initial infection [113-115]; these data are controversial. Pending further study, healthcare providers should encourage women to make such decisions based on their individual circumstances, values, and preferences.  
  
  Daniel J Sexton, MD
  Editor-in-Chief — Infectious Diseases Section
  Editor — Bacterial Infections
  Professor of Medicine
  Duke University Medical Center