20 CFR §404.1598

If a new severe impairment(s) begins in or before the month in which your last impairment(s) ends, we will find that your disability is continuing. The new impairment(s) need not be expected to last 12 months or to result in death, but it must be severe enough to keep you from doing substantial gainful activity, or severe enough so that you are still disabled under § 404.1594.