The first trimester begins before you’re pregnant. It starts on the first day of your last menstrual period and goes until the 13th week of pregnancy.
Your first trimester of pregnancy is full of many physical and emotional changes. It can be a very overwhelming time, and your mind may be racing with questions. Plus, your hormones are in overdrive. In fact, your body produces more estrogen during one pregnancy than it does during your entire life when you’re not pregnant. This surge in hormones can cause some unpleasant pregnancy symptoms. You may find yourself feeling moody, bloated, and tired. While you may not see a prominent baby bump yet, your uterus is expanding and your blood volume is increasing.
It’s OK to feel both excited and nervous. Talking to your friends, partner or a healthcare provider may help you feel better as you navigate your pregnancy journey.
The first trimester is so important because most of the fetus’s major organs and body systems are developing. Toxins, harmful substances and infection can severely damage a fetus’s growth and development during this time. It could increase your baby’s risk of being born with a congenital disorder.
Call your provider right away if you have:
Light bleeding or spotting during pregnancy is usually OK in the first few weeks of pregnancy. Some people experience implantation bleeding (when a fertilized egg burrows into your uterine lining). Call your pregnancy care provider if you’re bleeding heavily or the bleeding lasts more than one day.
Most healthcare providers recommend limiting caffeine consumption to under 200 milligrams per day during pregnancy. That’s about 12 ounces of coffee or three cans of Mtn Dew®. This is because a fetus can’t metabolize caffeine so it can build up in their body and cause complications.
Several developments occur in the first trimester. Although you can’t see it happening, there’s a lot going on inside your body after sperm fertilizes an egg.
During the first month of pregnancy, several important structures form. These structures are a tiny clump of cells, but will grow to become the amniotic sac, placenta and umbilical cord. A tube that becomes the fetus’s brain and spinal cord forms, as well as its circulatory system. A face, circles for eyes and the beginning of a mouth take shape.
The embryo is about a quarter-inch long — smaller than a grain of rice.
Several major organs begin to develop during the sixth week of pregnancy including the fetal lungs, heart, ears, arms and legs. Bones begin to replace tissue. Its head is large in proportion to the rest of its body, but it looks more human now. The fetus has a distinct mouth, nose and face. Some providers do an early ultrasound to confirm a heartbeat during this time.
By the end of the eighth week of pregnancy, the embryo becomes a fetus. It’s about 1 inch long or the size of a raspberry.
Towards the end of your first trimester, the fetus will have toes, fingers and nails. It will start to move by opening and closing its hands and mouth. The fetus’s urinary and digestive systems are also fully functioning. At around 12 weeks of pregnancy, your provider can listen to the fetal heart using a Doppler ultrasound. It also has either a vagina or a penis at this point (though your provider can’t see it on an ultrasound).
By the end of the 12th week of pregnancy, the fetus is between 3 and 4 inches long — about the size of a plum. It weighs about 1 ounce.
