Pregnancy gestation, by general definition, is the process of "carrying an embryo or fetus inside a female viviparous animal. A viviparous animal is an animal that doesn't lay eggs to reproduce and instead carries its young until a live birth. Humans fit into the category of viviparous animal. And like all viviparous animals, humans can have more than one gestation at the same time. In human gestation the gestational age of the fetus is determined from the last menstrual period (LMP) which adds two weeks to the gestational period. This isn't the case with animals and animal fetal development isn't determined by adding those two weeks.
The Nine Month Pregnancy
The human gestational period is approximately 266 days; almost nine months give or take a few days. Scientists have spent much time speculating why this amount of time for gestation. At first researchers thought that perhaps it had to do with optimal birthing time in early humans. But this connection was soon proved false because a baby conceived in the spring, when most mammals (excluding the modern human) tend to mate, would be born in the winter which was hardly the ideal time to present a vulnerable infant to the world. If this theory were correct, then the ideal human pregnancy gestation would be one year.
According to some studies, researchers recorded the gestational period of several viviparous animals and came to the conclusion that the size of the animal is directly connected to how long it takes for the baby to develop in the womb. African elephants, for example, have a 640-day gestation and Asian elephants have a 645-day gestation. Smaller monkeys like the Rhesus monkey have a 164-day gestation while it take a gorilla 257 days to grow a baby gorilla. Dogs and cats have a gestation period of approximately 60 days; rabbits can give birth every 33 days. Mice are even more prolific breeders and can have babies every 20 days or so.
How Far Along Are You?
Most pregnant women want to know how far along they are in their pregnancies and when they can expect to hold their new offspring in their arms. A gestation calculator can help you figure out these numbers. The Internet is full of these types of calendars and most are as accurate as the information you give them. They're also very easy to use. With the average gestation calculator, all you need to do is enter the month, day and year of your due date. Some calculators have drop down boxes for this. The online calculator will then tell you how many weeks you are into your pregnancy. Many will tell you how many weeks and days. The average month in a gestation calculator is 4.44 weeks.
Why Weeks Instead of Months
Estimated gestation is always done in weeks and there's a reason for this. A month is too long a period when any pregnancy-related medical decisions are necessary. People's way of interpreting months in pregnancy is different. For example, one woman may say she is five months pregnant when she is in the middle of her fifth month while another woman might only consider herself to be in the fifth months after that month is completed.
There's also an issue of precisely what is the length of a month. Weeks are always seven days long, but months can vary in length. Some are 31 days while others, like February, can be as short as 28 days. It's generally thought that a month is four weeks, but the difference in the number of days in various months disproves this theory. With this theory, a month should only be 28 days (four multiplied by seven days) which completely changes pregnancy calculations. The numbers can be manipulated to make them work, but really, who wants to? The math is simply too complicated.