Photo by wikipedia user Zerbey
Premature birth and, as a consequence, low birth weight are common problems for multiple births. Medically speaking any baby born before 37 weeks gestation is considered premature (40 weeks is considered full term) and a any baby weighing less than 5 pounds 8 ounces is considered low birth weight.
The lowest birth weight baby to be born in Tennessee and survive isn't a multiple though. Anyone want to guess how much she weighed when she was born 12 weeks early? Becca Hill weighed 13 ounces and was 9 inches long when she was born at Vanderbilt University Medical Center. It's worth stopping for a minute to ponder that. She weighed less than one pound. Less than an adult's hand. Around the same weight as your average paperback book (and probably half the weight of a Harry Potter book). According to the Tennessean article, Becca was a victim of 'placental insufficiency', which means just what it says. She wasn't getting what she needed in order to grow properly.
She had to spend 4 months in the Vanderbilt NICU, and it cost over $1 million in medical bills, but she made through and celebrated her first birthday back in the summer. She weighed about 13 pounds on her birthday, which is off the bottom of the chart for a 12 month old but some pretty phenomenal growth considering where she started. If the average newborn grew that much proportionally, he would be the size of a 15 year old boy by the end of his first year.
Developmental delays are pretty normal for premature birth babies. Doctors generally advise adjusting the normal milestones by the amount of time a baby was born early. So a baby born 12 weeks early would be expected to be three months behind her peers in development. Most premature babies do eventually catch up eventually.