1st, 2nd and 3rd Trimesters of Mom & Baby. Child growth in mother womb.

Overview

Pregnancy is measured in trimesters from the first day of your last menstrual period (LMP) and normally lasts about 40 weeks from conception to the birth of a baby. This time is roughly divided into 3 periods: the first trimester, second trimester and third trimester.

The Three Stages of Pregnancy
(1st, 2nd, and 3rd Trimester)

The first trimester is measured from conception to about the 12th week of pregnancy; the second trimester, from about 13 to 27 weeks of pregnancy; and the third trimester, from about 28 weeks of pregnancy until birth. Let's see what happens with you and your baby during each of the three stages.

First Trimester

Week 1 - Week 12

First Trimester: Early Changes in a Woman's Body

During the first trimester your body undergoes many changes. Hormonal changes affect almost every organ system in your body. These changes can trigger symptoms even in the very first weeks of pregnancy. Your period stopping is a clear sign that you are pregnant.

First Trimester: Physical and Emotional Changes a Woman May Experience

Other changes you may experience during the first trimester include:

• Extreme tiredness
• Tender, swollen breasts. Your nipples might also stick out.
• Upset stomach with or without throwing up (morning sickness)
• Cravings or distaste for certain foods
• Mood swings
• Constipation (trouble having bowel movements)
• Need to pass urine more often
• Headache
• Heartburn
• Weight gain or loss

First Trimester: Changes in a Woman's Daily Routine

As your body changes, you might need to make changes to your daily routine, such as going to bed earlier or eating frequent, small meals. Fortunately, most of these discomforts will go away as your pregnancy progresses. And some women might not feel any discomfort at all! If you have been pregnant before, you might feel differently this time around. Just as each woman is different, so is each pregnancy.

First Trimester: The Baby at 4 Weeks

• Your baby's brain and spinal cord have begun to form.
• The heart begins to form.
• Arm and leg buds appear.
• Your baby is now an embryo and 1/25 of an inch long.

First Trimester: The Baby at 8 Weeks

• All major organs and external body structures have begun to form.
• Your baby's heart beats with a regular rhythm.
• The arms and legs grow longer, and fingers and toes have begun to form.
• The sex organs begin to form.
• The eyes have moved forward on the face and eyelids have formed.
• The umbilical cord is clearly visible.
• At the end of 8 weeks, your baby is a fetus and looks more like a human. Your baby is nearly 1 inch long and weighs less than 1/8 of an ounce.

First Trimester: The Baby at 12 Weeks

• The nerves and muscles begin to work together. Your baby can make a fist.
• The external sex organs show if your baby is a boy or girl. A woman who has an ultrasound in the second trimester or later might be able to find out the baby's sex.
• Eyelids close to protect the developing eyes. They will not open again until the 28th week.
• Head growth has slowed, and your baby is much longer. Now, at about 3 inches long, your baby weighs almost an ounce.

Second Trimester

Week 13 - Week 28

Second Trimester: Changes a Woman May Experience

Most women find the second trimester of pregnancy easier than the first. But it is just as important to stay informed about your pregnancy during these months. You might notice that symptoms like nausea and fatigue are going away. But other new, more noticeable changes to your body are now happening. Your abdomen will expand as the baby continues to grow. And before this trimester is over, you will feel your baby beginning to move!

Second Trimester: The Baby at 16 Weeks

As your body changes to make room for your growing baby, you may have:

• Body aches, such as back, abdomen, groin, or thigh pain
• Stretch marks on your abdomen, breasts, thighs, or buttocks
• Darkening of the skin around your nipples
• A line on the skin running from belly button to pubic hairline
• Patches of darker skin, usually over the cheeks, forehead, nose, or upper lip. Patches often match on both sides of the face. This is sometimes called the mask of pregnancy.
• Numb or tingling hands, called carpal tunnel syndrome
• Itching on the abdomen, palms, and soles of the feet. (Call your doctor if you have nausea, loss of appetite, vomiting, jaundice or fatigue combined with itching. These can be signs of a serious liver problem.)
• Swelling of the ankles, fingers, and face. (If you notice any sudden or extreme swelling or if you gain a lot of weight really quickly, call your doctor right away. This could be a sign of preeclampsia.)

Second Trimester: The Baby at 16 Weeks

• Muscle tissue and bone continue to form, creating a more complete skeleton.
• Skin begins to form. You can nearly see through it.
• Meconium (mih-KOH-nee-uhm) develops in your baby's intestinal tract. This will be your baby's first bowel movement.
• Your baby makes sucking motions with the mouth (sucking reflex).
• Your baby reaches a length of about 4 to 5 inches and weighs almost 3 ounces.

Second Trimester: The Baby at 20 Weeks

• Your baby is more active. You might feel slight fluttering.
• Your baby is covered by fine, downy hair called lanugo (luh-NOO-goh) and a waxy coating called vernix. This protects the forming skin underneath.
• Eyebrows, eyelashes, fingernails, and toenails have formed. Your baby can even scratch itself.
• Your baby can hear and swallow.
• Now halfway through your pregnancy, your baby is about 6 inches long and weighs about 9 ounces.

Second Trimester: The Baby at 24 Weeks

• Bone marrow begins to make blood cells.
• Taste buds form on your baby's tongue.
• Footprints and fingerprints have formed.
• Real hair begins to grow on your baby's head.
• The lungs are formed, but do not work.
• The hand and startle reflex develop.
• Your baby sleeps and wakes regularly.
• If your baby is a boy, his testicles begin to move from the abdomen into the scrotum. If your baby is a girl, her uterus and ovaries are in place, and a lifetime supply of eggs have formed in the ovaries.
• Your baby stores fat and has gained quite a bit of weight. Now at about 12 inches long, your baby weighs about 1½ pounds.

Third Trimester

Week 29 - Week 40

Third Trimester: Changes a Woman May Experience

You're in the home stretch! Some of the same discomforts you had in your second trimester will continue. Plus, many women find breathing difficult and notice they have to go to the bathroom even more often. This is because the baby is getting bigger and it is putting more pressure on your organs. Don't worry, your baby is fine and these problems will lessen once you give birth.

Third Trimester: Emotional and Physical Changes a Woman May Experience

Some new body changes you might notice in the third trimester include:

• Shortness of breath
• Heartburn
• Swelling of the ankles, fingers, and face. (If you notice any sudden or extreme swelling or if you gain a lot of weight really quickly, call your doctor right away. This could be a sign of preeclampsia.)
• Hemorrhoids
• Tender breasts, which may leak a watery pre-milk called colostrum
• Your belly button may stick out
• Trouble sleeping
• The baby "dropping", or moving lower in your abdomen
• Contractions, which can be a sign of real or false labor

Third Trimester: Changes as the Due Date Approaches

As you near your due date, your cervix becomes thinner and softer (called effacing). This is a normal, natural process that helps the birth canal (vagina) to open during the birthing process. Your doctor will check your progress with a vaginal exam as you near your due date. Get excited — the final countdown has begun!

Third Trimester: The Baby at 32 Weeks

• Your baby's bones are fully formed, but still soft.
• Your baby's kicks and jabs are forceful.
• The eyes can open and close and sense changes in light.
• Lungs are not fully formed, but practice "breathing" movements occur.
• Your baby's body begins to store vital minerals, such as iron and calcium.
• Lanugo begins to fall off.
• Your baby is gaining weight quickly, about ½ pound a week. Now, your baby is about 15 to 17 inches long and weighs about 4 to 4½ pounds.

Third Trimester: The Baby at 36 Weeks

• The protective waxy coating called vernix gets thicker.
• Body fat increases. Your baby is getting bigger and bigger and has less space to move around. Movements are less forceful, but you will feel stretches and wiggles.
• Your baby is about 16 to 19 inches long and weighs about 6 to 6½ pounds.

Third Trimester: The Baby at 37 to 40 Weeks

• By the end of 37 weeks, your baby is considered full term. Your baby's organs are ready to function on their own.
• As you near your due date, your baby may turn into a head-down position for birth. Most babies "present" head down.
• At birth, your baby may weigh somewhere between 6 pounds, 2 ounces and 9 pounds, 2 ounces and be 19 to 21 inches long. Most full-term babies fall within these ranges. But healthy babies come in many different sizes.

Source: 

http://www.medicinenet.com/stages_of_pregnancy_pictures_slideshow/article.htm