Jessica Simpson's Baby Anxiety: Twins!

After all Jessica Simpson's "hot mess" years — through her divorce from Nick Lachey, and humiliation at the hands of John ("she's sexual napalm") Mayer and Tony Romo (who reportedly dumped her in an IHOP parking lot) — it's been heartening to see the pregnancy glow in the singer's eyes as she and fiancé Eric Johnson prepare for parenthood.

OK! NEWS: JESSICA SIMPSON DISHES ON HER WEIGHT: "I'M STILL CONFIDENT"

The star had fun with her pregnancy on Halloween, Tweeting, "It's true — I'm going to be a mummy!" along with a photo of herself in an Egyptian mummy costume, holding her bump.

But behind the punny facade, insiders say that as Jess' body grows exponentially, she's ridden with angst.

The biggest stressor? For a time Jessica, 31, who reportedly used IVF, was concerned that she might be carrying twins — at least.

OK! NEWS: JESSICA SIMPSON TWEETS PREGNANCY CONFIRMATION: "I'M GOING TO BE A MUMMY!"

"That was scary for her," says the source. "Jessica is a perfectionist and she was panicking that she wouldn't be able to cram in everything she needs to do in time for the birth.

"For instance, she put so much effort into making the nursery absolutely perfect ‚ she even had it repainted three times! So when the doctor said she might be having twins, she had to rethink everything."

Even the thought of having one baby is starting to weigh on Jessica.

OK! NEWS: JESSICA SIMPSON'S BABYMOON

"She's totally freaking out" an insider reveals to OK!. "Now that she's so close to the finish line, fear is setting in. She's been so excited to be a mom — it's been like a fairy tale, but it's becoming real and she worries she's not ready."

FOR MORE ON JESSICA'S PREGNANCY PANIC, PICK UP THIS WEEK'S ISSUE OF OK! — ON SALE EVERYWHERE NOW!