The SI unit prefix "pico" stands for multiplier 1x10-12 or 0.000,000,000,001.
Then 1 pg (pico-gram) = 1x10-12 gram = 0.000,000,000,001 gram, and
1 gram = 1x10+12 pg (pico-grams) = 1,000,000,000,000 pg (pico-grams).
The SI unit prefix "nano" stands for multiplier 1x10-9 or 0.000,000,001.
There are 1x10-9 or 0.000,000,001 grams in one nano-gram.
The following URL presents the "SI table of Unit prefixes:"
http://www.aqua-calc.com/page/si-units-prefixesBest regards,
Alex.