[Q.] A woman has many days remaining from Ramaḍān, can she begin fasting the six days of Shawwāl, in order not to miss Shawwāl?

[A.] No, she must complete Ramadān, due to his saying (ṣallAllāhu ʿalayhi wa sallam): ”Whoever fasts Ramaḍān then follows it with six days from Shawwāl…”, thus she has not fasted Ramaḍān so long as there remains upon her fasts from Ramaḍān.

Reference: http://www.alfawzan.af.org.sa/node/14891]