WASHINGTON -- The Smithsonian's National Zoo announced Thursday that its two six-week-old cheetah cubs, who survived a "rare and risky" birth, are gaining weight and doing well.

The cubs, being hand-raised by zoo staff since their harrowing birth in April, got a clean bill of health when they were examined by veterinarians this week. They are said to have put on about 3 pounds each -- they were about 1 pound each when they were born.

As you can see in the slideshow below, they certainly look bigger!

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  • National Zoo's New Cheetah Cubs

    The National Zoo's new cheetah cubs made their public debut on July 24, 2012.

  • National Zoo's Cheetah Cubs

    The National Zoo's new cheetah cubs made their public debut on July 24, 2012.

  • National Zoo's Cheetah Cubs

    The National Zoo's new cheetah cubs made their public debut on July 24, 2012.

  • National Zoo's Cheetah Cubs

    The National Zoo's new cheetah cubs made their public debut on July 24, 2012.

  • National Zoo's Cheetah Cubs

    The National Zoo's new cheetah cubs made their debut on July 24, 2012.

  • National Zoo's Cheetah Cubs

    The National Zoo's new cheetah cubs made their debut on July 24, 2012.

  • National Zoo's Cheetah Cubs

    The National Zoo's new cheetah cubs made their debut on July 24, 2012.

  • National Zoo's Cheetah Cubs

    The National Zoo's new cheetah cubs made their debut on July 24, 2012.

  • National Zoo's Cheetah Cubs

    The National Zoo's new cheetah cubs made their debut on July 24, 2012.

  • National Zoo's Cheetah Cubs

    The National Zoo's new cheetah cubs made their debut on July 24, 2012.

  • National Zoo's Cheetah Cubs

    The National Zoo's new cheetah cubs made their debut on July 24, 2012.

  • National Zoo's Cheetah Cubs

    The National Zoo's new cheetah cubs made their debut on July 24, 2012. Cheetah keepers Kathleen Baker (left) and Regina Bakely (right) attempt to aid the male down from a tree branch.

  • National Zoo's Cheetah Cubs

    Photographers wait for the unveiling of the cheetah cubs on July 24, 2012.

  • National Zoo's Cheetah Cubs

    Photographers shoot the unveiling of the cheetah cubs on July 24, 2012.

  • National Zoo's Cheetah Cubs

    The exhibit is open from 6 a.m. to 8 p.m. April to November.

  • National Zoo's Cheetah Cubs

    A six-week-old cheetah cub being bottle-fed.

  • National Zoo's Cheetah Cubs

    A six-week-old cheetah cub being bottle-fed.

  • National Zoo's Cheetah Cubs

    A six-week-old cheetah cub.

  • National Zoo's Cheetah Cubs

    A six-week-old cheetah cub.

  • National Zoo's Cheetah Cubs

    A six-week-old cheetah cub.

  • National Zoo's Cheetah Cubs

    Dr. Katharine Hope examines the National Zoo's 6-week-old female cheetah.

  • National Zoo's Cheetah Cubs

    Dr. Katharine Hope examines the National Zoo's 6-week-old male cheetah.

  • National Zoo Cheetah Cubs

    The National Zoo's cheetah cubs at 17 days old.

  • National Zoo Cheetah Cubs

    The cheetah cubs, at two days old.

  • National Zoo Cheetah Cubs

    The National Zoo's cheetah cubs at 2 days old.

  • National Zoo's Cheetah Cubs

    The National Zoo's cheetah cubs at 16 days old.

  • National Zoo's Cheetah Cubs

    The National Zoo's cheetah cubs at 16 days old.

  • National Zoo's Cheetah Cubs

    The National Zoo's cheetah cubs at 17 days old.

  • National Zoo's Cheetah Cubs

    The National Zoo's cheetah cubs at 2 days old.

The zoo said in a media release that the cubs are frisky, spending their time "running, jumping and chasing one another in their enclosure."

Wayne Millner, the cheetah keeper, told WTOP that they're "pretty goofy when they run because their bodies haven't caught up to their leg size."

The cheetahs are due to make their public debut this summer.