If you obtain your immigrant visa through consulate processing and the child is born after the US Embassy/Consulate issued your immigrant visa, the child may not need a visa to accompany you to the US. The Immigrant Visa Unit can prepare a letter for your child that can be presented to the Immigration Officer.
If you obtain your lawful permanent residency through USCIS and the child is born after you received the I-485 approval notice, the child is a US citizen by birth if born in the US. Otherwise, you need to file I-824 for your child. If the I-824 is approved, the USCIS will notify the US Embassy/Consulate that you are a lawful permanent resident so that your child can apply for a following-to-join immigrant visa.