After spending approximately 40 weeks inside the womb, the average newborn weighs 7 1/2 pounds. Most babies weigh between 5 1/2 and 10 pounds. The average length of a newborn is 20 inches (50 centimeters), with a range of 18 to 22 inches. The average head circumference (the distance around the head) is 14 inches (35 centimeters).
Lactobacillus and Bifidobacterium are the most common probiotic bacteria. The Lactobacillus genus has 18 different bacteria strains, while the Bifidobacterium genus consists of eight strains.
The two main types of bacteria considered to be probiotics include strains from the Lactobacillus and Bifidobacterium genera (other genera, such as Escherichia, Enterococcus and Saccharomyces have also been designated probiotics, but to a lesser and more questionable extent).