As a pregnant mother, you may spend plenty of time wondering about the growth and development of the life inside of your uterus. Your baby goes through a series of changes from the time of conception through birth. Most pregnancies last about 40 weeks. Your doctor may refer to your stage of pregnancy in terms of trimesters.

Conception

Although you may start calculating the length of your pregnancy from the first day of your last period, conception actually occurs about two weeks after the beginning of your menstrual flow. Zygote is the term for the fertilized egg cell. As the cell divides, it travels down your fallopian tube and implants into your uterus. Implantation occurs about six days after conception.

First Trimester

The first trimester of your pregnancy begins on the first day of your last period and last for the first 12 weeks of your pregnancy. As early as week three, your baby begins developing his brain, spinal cord and heart. Leg and arm buds begin forming around the fourth and fifth week of gestation, while his tooth buds and genitals appear between nine and 12 weeks. By the end of the first trimester, your baby measures approximately 3 inches in length and weighs almost 1 oz. At this point, he already has all his essential organs.

Second Trimester

At the beginning of the second trimester, your baby’s skin appears almost transparent. Fine hair forms on his head, while his liver and pancreas start to produce fluids. Your baby’s movements may feel like slight fluttering around the 18th week of your pregnancy. During the middle of your second trimester, bone marrow begins making red blood cells, while footprints and fingerprints form. By the end of the second trimester, your baby may weigh slightly over 1 lb and measure about 11 inches in length.

Third Trimester

Your baby finishes preparing for life outside your uterus during the third trimester. Your baby begins storing fat and minerals, such as calcium, iron and phosphorus, during this period of development. His eyelids open, and his lungs begin making breathing movements in preparation for his birth. His lungs continue to develop up to the time of his birth. During this final stage of pregnancy, your baby will most likely move into a head-down position. At the time of birth, your baby may weigh around 8 lbs and measure about 20 inches long.