The I.R.S. allows extra time for savers to take their first distribution. So people turning 70½ in 2018 can wait until April 1, 2019. But they must still take a second distribution, by Dec. 31 of next year, for 2019 — and take R.M.D.s annually after that, said Molly Stanifer, a financial adviser at Old Peak Finance in Chapel Hill, N.C.

It can sound confusing, but it pays to get it right, said Kacie Swartz, senior financial adviser at Stone Asset Management in Austin, Tex. Investors who fail to take an R.M.D. may face a steep penalty, equal to half the amount they didn’t withdraw. So if you were required to take out $10,000 but withdrew nothing, you could owe a penalty of $5,000.

Fidelity Investments said that as of late October, about half the company’s R.M.D.-eligible I.R.A. customers had not taken out any money toward their 2018 distribution.

That doesn’t necessarily mean, however, that they are forgetting to do so, said Joe Gaynor, director of retirement and income solutions at Fidelity. I.R.S. rules allow people with multiple I.R.A.s to calculate their minimum withdrawal on their total balance across the accounts, but then take a distribution from just one account (or split it across several). So it could be that people are taking money out of accounts held elsewhere, he said.

Here are some questions and answers about R.M.D.s:

Do I have to take an R.M.D. if I am still working?

Under I.R.S. rules, you must take distributions from I.R.A.s (except Roths) after you turn 70½, even if you’re still working.