Your gynecologist or obstetrician will measure your pregnancy in trimesters starting from the first day of the menstrual period before your conception, for a total of 40 weeks. The first trimester includes the complete embryonic stage and part of the fetus stage up to week 12, approximately 3 months long. The second trimester last until the 27th week, and the final trimester of your pregnancy will span from the 28th week until the birth of the newborn. Every week, the baby will continue to grow and develop up until the moment of delivery.
The First Trimester
In the first week after fertilization, the microscopic fertilized egg is growing from a few cells into a microscopic clump of cells (blastocyst), which will eventually implant into the walls of the uterus. It is during the implantation stage that the tiny cells will trigger physical and hormonal changes in your body.
Between weeks three and eight, the blastocyst continues to form during the embryonic stage and begin developing most body organs. It is at this stage that the embryo is extremely vulnerable to harmful substances including infectious diseases, radiation and alcohol.
By the ninth week, at one inch to 1 ½ inches in length, the embryo has developed into a fetus. At this stage, the uterus is now the size of a grapefruit or a clenched fist. The growing fetus has a resemblance closer to a tadpole with an extended tail and less like a human.
The Second Trimester
The fetus begins moving in the early stages of the second trimester, even though you likely will not notice it until 18 to 20 weeks into your pregnancy, when the movements become stronger.
The fatigue and nausea you likely felt during the first trimester have begun fading away. Even so, you are seeing other noticeable changes each week in your body. Your abdomen is now expanding substantially while the fetus continues to grow. As the fetus and you expand in size, you are likely feeling a variety of aches and discomfort including pain in the thigh, groin, abdomen and back. You are likely witnessing stretch marks appear on your thighs, buttocks, breasts and abdomen.
It is during the second trimester that the skin around your nipples begins to darken, and align begins to form running from your pubic hairline up to your belly button. Patches of dark skin will tend to appear on your forehead, cheeks, nose, upper lip, and on both sides of your face. Many gynecologists refer to this as a pregnancy mask. You may also experience carpal tunnel syndrome, where there is a tingling or numbing sensation in the hands.
The Third Trimester
At this stage, most body parts of the fetus have begun to develop and are now in the growing stage. By week 32, the fetus in your uterus is too large to move around easily. At the last of the third trimester, the fetus has usually calmed down, is moving less, and is positioned head down inside the uterus.
During this time, most mothers feel discomfort when the delivery is close at hand. In all likelihood, you will experience a heartburn sensation, shortness of breath, and swelling of the face, fingers and ankles. If extreme swelling suddenly appears or unexpected substantial weight gain occurs, it is important to call your doctor immediately, as this could be an indicator of preeclampsia.
In addition, many women experience tender breasts with leaking colostrum, a watery pre-milk solution, along with painful hemorrhoids. Your discomfort level might cause trouble sleeping, and you could experience a false or real labor contraction as your due date becomes closer.
In all likelihood, the obstetrician has used a variety of measuring tools to determine your due date, or the end of the 40th week of your pregnancy. However, delivering on the due date is simply a guideline, as many women deliver full term as early as the 37th week, and as late as the 42nd week.
Once the baby arrives, your pregnancy is finally over, and you can give away your maternity clothes. You will likely be looking forward to having your body transform back to its pre-pregnancy condition, and can finally get a full night sleep once again. However, as a new mom, you will likely have other concerns for a while.