Assalam o alaikum,

If a women is 3 or 4 months pregnant, does she have to fast in holly Ramadan?
If no, what are the conditions, Please answer me in details soon as possible.

Thank you for the great service as provided.

Fasting is fardh (obligatory) for adult Muslims, except those who are ill, travelling, pregnant, diabetic or going through menstrual bleeding.

Full details are in Wikipedia

Worth reading and talking to your doctor and a scholar: http://ramadanfastingandpregnancy.wordpress.com/ - lots of detail, gives Islamic and medical evidences and basically says its safe to fast if healthy and that minimum requirement is that she has to at least try then see how she goes

Fasting during ramadan is completely related to your will and faith plus your health.