"Conception is not totally impossible, but most unlikely," she says on her reality show

Credit: Peter Kramer/NBC

Although she’s been pregnant for much of her life and still has 18 kids at home – and is a grandma of three – Michelle Duggar remains hopeful she can have another child.

In an exclusive sneak peek of this week’s 19 Kids & Counting, Duggar, 47, opens up about visiting a fertility doctor with husband Jim Bob, 48, and getting blood work results.

“Conception is not totally impossible, but most unlikely,” she reveals in the clip, airing in Tuesday’s episode (9 p.m. ET) on TLC.

The Duggars, who have always insisted the number of children they have is in God’s hands, are parents to children ranging in age from 4 to 25, including five children below the age of 10.

Their youngest, Josie, was born prematurely but is now in good health. Duggar lost their 20th child to a a miscarriage in 2011. They mourned the little girl they named Jubilee Shalom, but told PEOPLE at that time they felt peace as well as deep sorrow.

“We’ve always put this in Gods hands,” Jim Bob says in the clip above of his and Michelle’s plan for more children, “and if God wants to give us more children, we are going to be happy either way.”