Martzen tearfully gave a Reedley detective her story of a 3 foot fall off a bed by Ella Vanleeuwen.
But what she didn't say to him could be important, and just taking the stand herself could also make a difference.
If one person can keep Megan Martzen out of prison, it may be Megan Martzen.
The 22-year-old faces a life sentence if she's convicted of killing 17-month-old Ella Vanleeuwen.
Two doctors have already testified that the toddler's injuries couldn't have happened from just the short fall from a bed Martzen described in a police interview jurors heard Friday.
They say it looked like Ella's head suffered a massive impact.
But Martzen's attorney says the babysitter was aware of another fall -- this one from a booster seat a day before the fall from Martzen's bed.
But Megan didn't tell this to the detective in the taped interview, and witnesses who were with Ella the day before say they never saw her fall from a booster seat.
ABC30's legal analyst Tony Capozzi said, "If there's no evidence or no witness that can say that this happened, the evidence is irrelevant. It doesn't come in. But the smart defense lawyer will try to implant in the jury's mind that this happened."
Capozzi says Martzen can't testify to something she didn't see happen. Her attorney can use doctors to pose a scenario where multiple falls, not Megan Martzen killed the little girl.
Capozzi also says Martzen's tearful accounting to police of the last conscious moments of Ella's life may put her in the jury's good graces.
The Reedley detective is expected to be the last witness for the prosecution.
He'll wrap up on Monday, then the defense takes over, and Martzen's attorney said he does expect his client to testify.
"The jury's going to develop some sympathy with this young woman and if she takes the stand and comes off as a credible witness, she may create the reasonable doubt to the jury," Capozzi said.