Mr Seymour lobbied the Government for the changes after he agreed to vote against a previous member's bill sponsored by Labour MP Sue Moroney that would have extended leave for such families.
Announcing the changes, Workplace Relations and Safety Minister Michael Woodhouse said they would cover pre-term babies only because extra assistance was already available for parents of multiple-birth children and children with disabilities.
That had "made a mug" of Mr Seymour, Ms Moroney said.
"The watered-down version David Seymour has been forced into will leave an additional 1000 families each year with inadequate paid leave when they have a special needs baby or a multiple birth."
That was rejected by Mr Seymour, who said he had taken on new information and revised his position.
"In the case of multiple births you get additional carers to help look after multiple babies, that frees up parents' time with children ... if you look at additional benefits for people with babies with special needs, you have Working for Families credits, for instance.
"Is it a per week benefit such as paid parental leave - no, it's not. Does it allow parents to spend more time with their children - yes, it does."
Ms Moroney's new member's bill to extend paid parental leave to 26 weeks will be before Parliament next week. Labour, NZ First, the Greens, and the Maori Party will vote in favour of the bill, meaning United Future leader Peter Dunne will have the deciding vote. He has expressed support for 26 weeks, but has refused to say which way he will vote.
National could veto the changes on the basis it would cost too much.
This year the Government increased paid parental leave from 14 to 16 weeks. A further lift to 18 weeks is due next year.