Mudgalapurana 3.21.13

3.21.13

mṛte bhartari patnī sā garbhayuktā rajasvalā |
tadā patyā na saṅgacchedvidhavādharmagā bhavet ||

If the wife is pregnant or menstruating when her husband dies, / then she should not accompany her husband; she should abide by the duty of a widow.