Windows Defender doesn't protect against the Wannacry ransomware, according to Michael Horowitz from Computerworld. Microsoft announced on the 12th of May that Windows Defender would be able to detect and remove Wannacry, but apparently that wasn't valid for Windows 7. Horowitz claims to have the information from sources close Microsoft.
The 12 May announcement mentioned that Microsoft updated Windows Defender to deal with Wannacry. However, Defender is only an anti-spyware tool on Windows 7. That's different from Windows 8.1. and Windows 10, where Defender functions as a full fledged antivirus product.
Users on Windows 7 that want to protect themselves against Wannacry (and they should, 98% of the Wannacry infections were on this OS) should therefore not rely on Windows Defender, instead they should get Microsoft Security Essentials. That is because the update that Microsoft rolled-out is for Windows Defender on Windows 8.1 and Windows 10 and Security Essentials for Windows 7 and Vista.
Users should therefore use either Security Essentials on Windows 7 or a third-party antivirus scanner to protect themselves against Wannarcry.
