In a televised address to the nation on New Year's Eve, Modi had announced pan-India expansion of the maternity benefit programme which was being implemented on a pilot-basis across 56 districts.
"On 31 December 2016, Prime Minister Narendra Modi finally announced that maternity benefits would be provided very soon. One year later, however, the new scheme framed for this purpose (Pradhan Mantri Matru Vandana Yojana) is yet to be operationalised," according to the letter on December 20.
In May, the Union Cabinet approved the Pradhan Mantri Matru Vandana Yojana for pregnant women and lactating mothers. Under the scheme, a woman would receive Rs 5,000 in three installments upon meeting different requirements and remaining Rs 1,000 under Janani Suraksha Yojana after institutional delivery.
The benefits would be offered only for the birth of the first living child.
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The letter also points out that restricting the benefits to only the first-born is in violation of the National Food Security Act (NFSA).
The economists have also appealed that the government set aside sufficient funds in next year's budget for the scheme as Rs 2,700 crore for the programme as per the 2016-2017 budget is a third of what is required based on norms under National Food Security Act.
The letter also demands that the central government's contribution to old-age pensions under the National Old Age Pension Scheme (NOAPS) scheme should be raised from Rs 200 per month to Rs 500.
Maternity benefits became a legal entitlement in 2013 under the National Food Security Act which states that "Subject to such schemes as may be framed by the central government, every pregnant and lactating mother shall be entitled to [nutritious food and] maternity benefit of not less than rupees six thousand, in such instalments as may be prescribed by the central government".