Your doctor has confirmed you are carrying twins and the excitement and anticipation you felt when you found out one baby is coming has just been increased exponentially.

There are many questions that will remain with you throughout your pregnancy: 'What will they look like when they're born' is probably the most common thought you'll have. Will they be healthy? Whose eyes will they have?

In terms of your pregnancy development, you can add these questions to the mix as well: At what stage are they now? What are they doing in there? How big are they?

Let's solve some of the mystery for you now.

First Trimester

During this time in your conception, you will suspect that you are pregnant. You will probably have a missed period and experienced some of the classic symptoms of morning sickness; vomiting, nausea and fatigue to name a few. At this point in your pregnancy, your symptoms will still mimic that of a singleton pregnancy.

The babies will start of as a ball of cells or a blastocyst, which is the scientific name, for the first 3 weeks.

By six weeks you are probably just confirming the pregnancy, though some mothers find out later. Now, the babies are about a quarter-inch long and their head is much bigger than the rest of their body. A lot is happening by the end of the six weeks. You may be able to see their hearts beating if you have an ultrasound at this point. Don't worry if you haven't by six weeks, it's still considered early in the pregnancy and your doctor simply may not have yet scheduled it.

The babies have developed the dark spots where their eyes will form, the beginnings of their arms and legs have appeared and their brains and lungs have begun developing. Chances are you don't know you're carrying twins and all of this is still going on inside you.

By the end of the trimester, you are twelve weeks pregnant. Somewhere between 5 weeks and the twelve week mark, there is an excellent chance you have found out you are carrying twins. You probably had an ultrasound to confirm your doctor's and maybe your own suspicions.

What's going on with the babies at this stage of their development? A lot is happening in your womb by the end of the first twelve weeks. The babies' brains and spinal cord continue to progress. Their ears, nose and feet begin to form and they have begun to move, though it is still too early for you to feel their movement, even with two babies growing inside you. Though their eyes remain shut and still forming, they now have eye color.

By the end of twelve weeks, the babies have grown to a length of approximately 2 inches and weigh about 4 to 5 ounces each.

Keep in mind that all weights, measurements and time-lines given in this article are averages for a twin gestation. Every pregnancy is unique, even within the same mother.