Health insurance through work locks for the plan year. You pick during open enrollment and the elections hold until the next one. The IRS is the reason. Those premiums come out of your pay before taxes, and the tradeoff for that tax break is that you can't change your mind in March.
There's a list of exceptions. Your plan calls them qualifying life events, which means a change in your life big enough that the IRS lets you redo the election mid-year. Six show up most often.
Marriage or divorce. Getting married lets you add a spouse and stepchildren. A divorce or legal separation lets you drop them, and usually requires you to, because an ex-spouse is not a dependent.
A birth or an adoption. Coverage for a newborn is generally retroactive to the date of birth as long as you report it inside the window. Miss the window and the hospital bill lands on you.
Losing other coverage. Your spouse loses a job and the family plan goes with it. A kid turns 26 and ages off. A COBRA period runs out. Losing coverage counts. Dropping it on purpose, or losing it because you stopped paying the premium, usually doesn't.
A change in employment status that changes eligibility. Part time to full time, or the reverse. Unpaid leave. A strike or a layoff.
A move that takes you out of the plan's service area. This one matters most on an HMO, where the network is geographic, and matters less on a national PPO.
A change in your spouse's open enrollment. If their employer runs a plan year that starts in July and they change coverage then, you can usually adjust yours to match.
The window is short, and missing it costs real money. Most employer plans give you 30 days from the event to report it and make the change. A couple of situations get 60, including gaining or losing Medicaid or CHIP eligibility. Thirty days runs out fast when there's a new baby in the house, so put it on the list alongside the birth certificate and the Social Security card.
Some things that feel like qualifying events are not. Your doctor leaving the network doesn't qualify. Your premium going up mid-year doesn't. Realizing in April that you picked the wrong plan doesn't, and that one comes up more than the other two put together. The high-deductible plan that looked cheap in November is the one people want out of by spring, and the answer is no until the next open enrollment.
Your FSA election can usually be changed alongside the health election when one of these events happens, and a dependent care FSA can change when daycare costs or providers change. An HSA contribution can be changed any month you feel like it, because an HSA isn't a plan-year election in the first place.
Call HR the week it happens, not the month. Ask two things: what's my deadline in writing, and what paperwork proves the event. A marriage certificate, a birth certificate, or a letter showing the date other coverage ended is usually all they need. Once the window is open and you're comparing two plans, deductible, copay, coinsurance, out-of-pocket max lays out which number you actually end up paying.