Following your pregnancy week by week is always helpful but, unless you are sure of the date of conception, may not be entirely accurate. Here is a study of a typical pregnancy week by week.

Week one is, in many ways, the most important week in your week by week pregnancy calendar. Without week one, the other weeks would not happen. There would be no baby. In week one the man's sperm meets the woman's egg inside the fallopian tube and the life of your baby begin. While many sperm may penetrate the outer layer of the egg only one sperm gets past there into the ovum and fertilizes it. Inside your ovum is where yours and the male's chromosomes start to form the baby's distinct chromosomal makeup. The exact moment of conception determines your baby's gender. This is decided by the sperm.

Should it happen that two eggs ret released from your ovaries, or one egg is released from each ovary, and these get fertilized by the male's sperm but two separate ones, you've conceived fraternal twins. If only one egg is fertilized but it later is divided into two, you'll have identical twins.

Identical twins have the same combination of chromosomes and are therefore the same gender. Their hair and eye color will be the same as will their blood types. Two thirds of all twins are fraternal, however.

The first week of this week by week pregnancy pattern you may experience some vaginal bleeding, or slight spotting. This happens at the same time your baby travels from the fallopian tube and lands in your uterus. It's a perfectly normal occurrence and if you should happen to be aware of it will give you a good indication that this is week one of your week by week pregnancy schedule.

Of course, the second most important week of your week by week pregnancy schedule is the week your baby is born. The week before you give birth the baby has shed most of the fine downy hair on its body known as lanugo. Her or his lungs are nearly mature and their capacity for breathing on their own is increasing - known as surfactant production. Your baby is too big to move around very much in your womb at this point. Generally you have a seven pound baby by this time. It has settled into your pelvis and you may feel a little off balance at this change. While you can now breathe easier than in the prior weeks of this week by week pregnancy schedule, you're urinating more as the baby is pressing on your bladder more. Your uterus is now at least 6 ½ inches higher than your navel.

The last week, week 40, 15 percent of the baby's body is fat - to keep it warm - and he or she weighs approximately 7 ½ pounds. The baby has shed most of its vermix, oil that covers the baby's skin to protect it from the waters in the womb. The baby's chest sticks out. It's week 40 and you're about to have a baby.