Dear member, it is not clear from the facts mentioned by you since when contributions under the ESI Act, 1948, and the rules/regulations framed thereunder have been paid by your employer in your case. When you joined the organization in January 2015, it is unclear whether contributions have been paid from January 2015 itself. If contributions were paid from April 2015 onward only, then why the gap?
Further, I am doubtful whether you are eligible for maternity benefits under the ESI Act in December 2015 (the due date as mentioned by you) or prior to this period. The reason being that "an insured woman shall be qualified to claim maternity benefits for a confinement occurring or expected to occur in a benefit period if the contributions in respect of her were payable for not less than seventy days in the immediately preceding two consecutive contribution periods" (Section 56(1) of the said Act). If you started contributing w.e.f. 01/04/2015, the first benefit period in your case will start from 01/01/2016 only (though you can get only medical treatment prior to this date). Please see Regulation 4 of the ESI (General) Regulations, 1950.
I will, however, suggest you visit your appropriate Branch Office of ESIC to which you are attached and discuss the issue with the Branch Manager concerned so that you may be able to get the correct position regarding your eligibility for the said benefit. While visiting the Branch Office, please ensure you have your Pehchan Card (as issued by the Branch Office, ESIC). Furthermore, I hope you will also ensure to get proper medical certification from the ESIC Dispensary to which you are attached.