The Three Trimesters Obstetrics & Gynecology
Pregnancy
the three trimesters
Pregnancy has three trimesters, each of which is marked by specific fetal developments. A pregnancy is considered full-term at 40 weeks; infants delivered before the end of week 37 are considered premature. Premature infants may have problems with their growth and development, as well as difficulties in breathing and digesting.
First Trimester (0 to 13 Weeks)
The first trimester is the most crucial to your baby's development. During this period, your baby's body structure and organ systems develop. Most miscarriages and birth defects occur during this period.
Your body also undergoes major changes during the first trimester. These changes often cause a variety of symptoms, including nausea, fatigue, breast tenderness and frequent urination. Although these are common pregnancy symptoms, every woman has a different experience. For example, while some may experience an increased energy level during this period, others may feel very tired and emotional.
Second Trimester (14 to 26 Weeks)
The second trimester of pregnancy is often called the "golden period" because many of the unpleasant effects of early pregnancy disappear. During the second trimester, you're likely to experience decreased nausea, better sleep patterns and an increased energy level. However, you may experience a whole new set of symptoms, such as back pain, abdominal pain, leg cramps, constipation and heartburn.
Somewhere between 16 weeks and 20 weeks, you may feel your baby's first fluttering movements.
Third Trimester (27 to 40 Weeks)
You have now reached your final stretch of pregnancy and are probably very excited and anxious for the birth of your baby. Some of the physical symptoms you may experience during this period include shortness of breath, hemorrhoids, urinary incontinence, varicose veins and sleeping problems. Many of these symptoms arise from the increase in the size of your uterus, which expands from approximately 2 ounces before pregnancy to 2.5 pounds at the time of birth.
UCSF Health medical specialists have reviewed this information. It is for educational purposes only and is not intended to replace the advice of your doctor or other health care provider. We encourage you to discuss any questions or concerns you may have with your provider.
Your pregnancy: weeks 13 to 16
Your baby
Your baby has more muscle tissue and harder, more developed bones. Skin is beginning to form, but it's almost transparent at this point. Your little one is starting to roll, kick, and move around a lot — flexing tiny arms and legs, too.
What’s happening this week?
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Week 13
A plum
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Week 14
A lemon
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Week 15
An avocado
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Week 16
An apple
Week 13 A plum
13 weeks pregnant
Hello, second trimester! Goodbye, morning sickness. (I hope!)
- Your baby is almost the size of a plum.
- Long, thin arms are growing.
- Your little one’s tiny hands can make a fist.
- Eyelids are formed and closed, to protect developing eyes.
Week 14 A lemon
14 weeks pregnant
By now my baby can suck their thumb. Hope it doesn’t become a habit!
- Your baby is the size of a lemon now.
- Your little one is very active — moving all the time.
- Your baby is busy smiling, frowning, and making other facial expressions.
- Hair is growing on your baby’s head and eyebrows.
Week 15 An avocado
15 weeks pregnant
Hard to believe I can love something this tiny so much!
- Your baby is the size of an avocado.
- They've has grown a whole inch since last week!
- Now your baby can swallow, suck, and breathe in amniotic fluid.
- Your little one can curl their tiny fingers and toes.
Week 16 An apple
16 weeks pregnant
The apple of my eye!
- Your baby is the size of an apple and continues to grow quickly.
- Your little one is moving around inside the amniotic sac.
- Your baby is strong enough to hold up their little head.
Your body
Your baby bump might be starting to show, and your clothes may be getting a little snug. Hopefully, you’re feeling good and have more energy and less morning sickness than you did in your first trimester. For some, it takes a bit longer to feel better.
Your to-do list
- If you work, start thinking about when you plan to go on maternity leave. Talk to your employer about your company’s maternity policy and other benefits.
- Keep wearing your seat belt, even as your belly expands. Stay comfy and keep baby safe by placing the lap band below your baby bump.
- Try to find 10 minutes to relax every single day. Growing a baby is hard work!