Yes, you may be able to get the extension. I would document the reasons and have an immigration attorney file the necessary extension paperwork for you to get it done right. Let me know if my office can help. By the way, congrats on upcoming new addition to your family.
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Your mother can file her own application to extend her stay in the U.S. with adequate justification, intent to return to her home country at the conclusion of the requested period of extension (no more than 180 days), evidence that the applicant has adequate funds to support her extended visit in the U.S. or is otherwise staying in c/o a friend or family member, evidence indicating ties to the home country, etc. If not handled properly, there could be status violations, invalidation of existing visa, finding of unlawful presence, removal from the USCIS, etc.
The information contained in this answer is provided for informational purposes only, and should not be construed as legal advice on any subject matter, nor should it be viewed as establishing an attorney client relationship of any kind.
Congratulations on the new addition to your family! Your mother should be able to request an extension of her visa for up to another 6 months via application to USCIS. When she sends in the extension application, she'll need to be sure to include information and documentation showing that there's a good justification for the extension (ie, that your wife needs help after delivery and you'll be working), that she maintains strong ties to her home country and intends to return at the end of her visit, and that she will be able to support herself in the U.S. without working (either by showing that she has adequate funds to pay for her stay, or that she has family in the U.S. that she will be staying with).