OK, since you are a Canadian citizen, your child will be a Canadian citizen (actually a dual US/Canadian citizen) - whether born in the States or in Canada. If the child is born in the States (which it sounds like s/he will be), you're going to need to get him/her a
citizenship certificate so that s/he can come to Canada with you in April. If your child is born in Canada - even before your husband lands - he won't technically have to "declare" the birth at landing . . . but whether born in the States or in Canada, it will be okay to say, when they ask, that he has had a Canadian citizen child just born.
The
whole reason behind declaring any new dependents not included on the PR application is because any family member not declared on landing who might need to be sponsored for permanent status at a later date needs to be declared before the immigrant lands, or they are forever barred from sponsoring them. In other words, they don't like immigrants to "hide" family members that might make them inadmissible. But a child born to you (a Canadian citizen) and your husband will not ever need to be sponsored for permanent status - so although you are going to have dealings with bringing the child into Canada (the need for the citizenship certificate), your husband is not under any obligation to "report" the birth at his landing. But there's no harm is disclosing it either - as long as the IO is clear about the fact that this is a Canadian citizen child and not in need of future sponsorship.