There's a secret behind JANET JACKSON's dramatic weight loss — marriage and children!

The 45-year-old pop star has dropped 30 pounds on the Nutrisystem diet over a five-month period, and The ENQUIRER has learned that her motivation for shedding the weight was to slim down for a late summer wedding to billionaire Qatari businessman WISSAM AL MANA.

What's more, Janet and Wissam, 36, are making arrangements to raise her beloved late brother MICHAEL's children — PRINCE, 15, PARIS, 14, and BLANKET, 10 — once her elderly mother KATHERINE can no longer care for them.

"Janet and Wissam have been planning to marry for some time, but she wanted to fit into the perfect wedding dress," said a family insider. "And once she and Wissam are married and settled, she wants to take custody of Michael's children."

The 5-foot-4 singer/actress, who now weighs a svelte 118 pounds, has seen her weight soar as high as 180 over the past few years.

She has admitted that she packed on the pounds after Michael's tragic death in 2009. And just a few months ago, she was tipping the scales at 150. But after finally getting past the grief of losing Michael and the involuntary manslaughter trial of his physician, Dr. CONRAD MURRAY, Janet "decided to make a change in her life" and lose the weight, said the insider.

Janet and Wissam had originally planned on having their own children, but after much consideration they decided that their best option would be take custody of Michael's kids. "Janet knows that there are huge risks in having children at her age," noted the insider. "She also wants this slim-down to be her last, and a pregnancy would cause her to gain a lot of weight again.

"Wissam adores Janet and agreed to go along with whatever she wants.

"Janet has a healthy new lease on life and is ready for anything, especially marriage and motherhood."