megan huntsman , 40 , pleaded guilty to the six murders in february .
police said children , born between 1996 and 2006 , were alive just minutes .
she would strangle newborns with her thumbs and hide them in garage .
two of her old daughters spoke up for their mom in letters read in court .
huntsman , from pleasant grove , utah , was sentenced for her crimes today .
given six sentences of five years to life , three of which are consecutive .
means she will spend at least 15 years behind bars , and likely much longer .