Those adorable newborn puppies that you can't stop cooing over aren't well equipped to be outdoors yet. Their bodies don't retain heat well, and they rely on their mom's body heat for much of the first couple of weeks. When they're a little older -- about 4 weeks old -- and more in control of their bodies' functions, it's safe to take quick jaunts outside as long as the weather is favorable.
First Two Weeks
During the first two weeks of life, puppies can't see, aren't coordinated enough for much movement other than crawling closer to mom and are unable to regulate their body temperatures. If you take them outside, they could quickly succumb to the elements -- even if it's warm. They'll be afraid to be away from littermates and their mom, and even the slightest breeze could quickly lower body temperatures. The ASPCA recommends letting the mom handle most of the care during this time; you should even limit the amount you touch and handle the puppies until at least week three.
Weeks Three and Four
Puppies start gaining independence during their third and fourth weeks. Their eyes are open and they are moving more confidently. During this time, you should start handling the puppies carefully to help socialize them to people. At 4 weeks old, the puppies are able to start the weaning process, taking less nourishment from mom and moving toward eating gruel and solid food. With solid food typically comes more pronounced potty need; mom licks the puppies to stimulate urination and bowel movements through week three, but from that point, the puppies should be able to go on their own. You can start taking them outside for short periods at 4 weeks old after they eat to see if they will go potty and poop in the yard. Do this only if the weather is warm enough that you don't need a jacket to be outside; the puppies can regulate their body temperatures better at this point, but a drastic temperature difference can still affect them.
By 6 weeks old, the puppies should be well on their way to finishing the weaning process, and they're romping and playing whenever they're not sleeping. They gained some disease immunity from their mother's milk, but it's time to take them to the vet for the first round of vaccinations. It's best not to take them around other dogs – especially dogs you aren't familiar with – until these shots kick in. So for this trip, head straight to the vet and then back home, keeping your puppies as separate as possible from other dogs in the vet's waiting room.
Although it can be dangerous to remove newborn puppies from their mother's care and take them outside, sometimes you must. When a puppy isn't thriving, you must get it to the vet immediately to diagnose and treat potential problems. Warning signs include vomiting, low weight gain, coughing, pale gums, constant crying and swollen eyes. Wrap the puppy up in a blanket, preferably with a simple heat source such as a warm water bottle, to keep his temperature up while you're transporting him.