Pregnancy Calendar: 1st Trimester


Your breasts ache, standing in checkout lines makes you faint and you have to pee all the time. Sounds like fun? You might be in the first trimester of pregnancy. From the moment a sperm enters an egg, amazing things begin to happen. Although it will be months before it can be felt kicking, in the first trimester of pregnancy the tiny passenger will make itself known to the mother in typical ways.


At about for weeks, the embryo sends out hormones that cause the monthly menses to cease (although there can be some slight spotting as the fertilized egg attaches itself to the uterine wall). Morning sickness is common in the first trimester of pregnancy; an easy cure is to munch a few saltine crackers before getting out of bed. The first trimester of pregnancy can also bring a heightened sensitivity to smells. A woman who smokes tobacco may find herself ill at the mere thought- a good thing, really, as this is a perfect time to quit.

Aside from the weird symptoms the mother is experiencing, all sorts of things are happening, unseen, within her body. This first trimester of pregnancy is a time of tremendous change. From the unlikely meeting of egg and sperm (millions are sent, few hit their mark) chemicals are produced that sustain the pregnancy. If something goes awry early in the first trimester of pregnancy, the mother may slough off the resulting tissue believing she is only having a regular -if oddly timed, menstrual period.

When everything goes right, the first trimester of pregnancy sees the cells change from zygote to embryo, and from an infinitesimal speck to an inch-long creature with eyelids and fingers. Five weeks into this trimester, the developing embryo has consumed its yolk and the placenta begins functioning as its source of nutrition. At this point, the mother truly is 'eating for two'.

All things considered, the first trimester of pregnancy is a time of mysterious change in the dark and secret space of the womb. The first trimester of pregnancy can be magical - try to remember that when you're wide awake and peeing for the fifth time tonight.

Week by Week

Pregnancy is an exciting time for any woman. Your body is changing, your baby is growing and developing, and you are preparing for the most rewarding experience of your life. Here is a weekly pregnancy calendar to the changes that you will experience in your first trimester, as well as how your baby is developing each week.

The first trimester includes Weeks 1-13. During this time you will notice the first signs and symptoms of pregnancy and will begin to become used to the changes that are occurring. Enjoy your first trimester pregnancy calendar!

Week 1 - Week 2

The beginning of your pregnancy is calculated from the beginning of your last period. So technically during this time, you have not conceived yet. This is the start of the gestational period which is the traditional 40 weeks used to determine a pregnancy length. This period will determine your due date.

Week 3

This is where you most likely have conceived your baby. You may not notice anything changes yet because you probably do not yet know that you are pregnant. The embryo inside you is only about 0.006 inch long.

Week 4

You will most likely miss your period around the end of this week. You may also begin to notice more signs of pregnancy. The fetus is around 1 mm in length and the amniotic cavity and placenta are forming. Layers of cells called germ cells are developing. Each layer will become an important part of the major body systems - nervous system, digestive system and musculoskeletal system.

Week 5

Most likely you have skipped your period and now would be the time to have a pregnancy test. Although you can buy a urine test kit at your local store, your health care provider's office may offer a blood test which could be more accurate this early. At this point you may have to urinate more frequently and may tire more easily. Your baby is about 1/2 inch long and the plate that will become the heart has developed. The central nervous system and skeleton are starting to form as well.


Week 6

It is possible that you have begun gaining weight. Do not panic! It most likely isn't much. If you have experienced nausea or vomiting, you may have lost weight. Again, do not panic! This is also normal. At this point you should have a pelvic exam. From the top of your baby's head to its buttocks (called crown-to-rump length) is about 2 to 4 mm. This begins the critical growth and development period, so you want to be careful about environmental factors that may negatively affect you and your baby.

Week 7

By now you have probably gained a couple of pounds, though again it isn't unusual if you have lost weight. Your baby is about 4 to 5 mm this week. The leg buds and arm buds are beginning to appear. The heart has divided into left and right chambers, bronchi (air passages in the lungs) are present, and the brain is growing.

Week 8

Your uterus is starting to get bigger but you probably aren't showing yet. You may begin experiencing pain caused by the expanding uterus pressing on the sciatic nerve. The best solution is to lie on the side opposite to where the pain is. Your baby is approximately 14 to 20 mm crown to rump. Eyelids and ears form and the aortic and pulmonary valves of the heart are already distinct.

Week 9

You will most likely not notice any new changes this week. Your baby, however, is now 22 to 30 mm crown to rump. The arms and legs are longer, and the head is more erect with the neck more developed. The baby will move, though you will not feel it - this can be detected on an ultrasound.

Week 10

By this time, you are probably feeling a lot of conflicting emotions or experiencing the beginning of mood swings. This is due to the changing hormones caused by your pregnancy. Your baby's crown-to-rump length is about 31 to 42 mm and is starting to put on a little weight.

Week 11

Your uterus now is almost big enough to fill your pelvis. You may start to notice subtle changes in your hair and nails. There are speculations that this is caused by increased blood circulation, others say it is due to hormonal changes. Your baby is about 44 to 60 mm crown-to-rump and weighs about 0.3 ounce. Your baby's head is almost half of the entire length and the neck is lengthening. External genitalia are beginning to show though development complete won't for another 3 weeks. If you haven't already, you should probably have your first ultrasound now.

Week 12

By the end of this week, you may feel your uterus above your pubic bone since the uterus is growing upward. Your baby weighs about half an ounce and is almost 2.5 inches long. It is around this time that you may be able to hear the baby's heartbeat with a provider's doppler machine. All of the baby's vital systems have formed by this week, but they will continue to grow and develop throughout the rest of your pregnancy.

Week 13

Hooray! You have reached the last week of your first trimester. If you have experienced morning sickness or have not felt well, you will most likely begin feeling better. Your baby is about the size of a peach by now.

This pregnancy calendar for your first trimester should be used as a guide and should not substitute for a health care provider's care.

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