The First Trimester

The first trimester encompasses weeks zero through twelve, the second trimester will include weeks 13 through 27 and the third trimester will cover from week 28 up to the birth of your baby. This stage of pregnancy is crucial to the development of the fetus. Although you may not look pregnant, there are major changes occurring to both the mother and the fetus. For the first eight weeks, the fetus is called an embryo. This embryo rapidly develops into a fully formed fetus. The first trimester is when the fetus is most vulnerable as all of the major organs and body systems are forming. It’s important that alcohol, tobacco, drugs and radiation are avoided.

The Embryonic Stage

During the week after fertilization, the fertilized egg will implant into the uterine wall. In the first four weeks all of the major organs and systems will begin to form. The neural tube, which becomes the spinal cord and brain begins to form. It’s important that the mother intakes enough folic acid to prevent any defects from occurring in the neural tube. The eyes and ears are beginning to develop and tiny limb buds appear that will eventually develop into arms and legs. The embryo will have a heartbeat, but it is too faint to detect yet.
Weeks four through eight complete the embryonic stage. The embryo will continue developing and functioning all major body systems. It will start taking on a more human shape, although the head will be much larger proportionately to the rest of the body. The arms and legs will now be clearly visible. The mouth will be developing tooth buds which will eventually become baby teeth. The embryo’s facial features, such as eyes, nose, mouth and ears are all becoming more distinct. Fingers and toes will still be webbed and bones are beginning to develop. Around eight weeks, mothers often have their first ultrasound, where they will also get to hear the baby’s heartbeat using a doppler. After the eighth week, the embryo officially becomes a fetus.

Development of the Fetus

Weeks nine through twelve complete the first trimester. During these weeks the fetus will develop fingernails and toenails. It will not have eyelids and fully formed arms and legs. The larynx, or voice box, will begin to develop. The external genital organs will also have developed, although the parents will usually not find out the baby’s sex until the 20 week ultrasound. Now, at the close of the first trimester, the fetus will typically be 3-4 inches long and weigh only half an ounce.

Changes in the Mother

Pregnancy is different for each mother. Some may experience many symptoms or none at all and these symptoms can last for varying lengths for each person. Some of the common first trimester symptoms include nausea, breast tenderness, fatigue, constipation, discharge, food craving and aversions, heartburn, mood swings and more. Light spotting can also occur, but if you experience heavy bleeding, cramping or severe abdominal pain, call your doctor immediately as these can be signs of miscarriage or ectopic pregnancy. In the first few weeks of pregnancy, your hormone levels will be changing significantly. In the first trimester, your uterus begins to expand to support the growth of the placenta and the fetus. Your uterus will have an increased blood supply to carry oxygen and nutrients to your developing baby.

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About Coffee and Wine Mom

Maddi Casey is a muscle car driving, hard working mama who is obsessed with football and good beer. She has two beautiful baby girls and loving husband. Writing is a passion, hobby and sometimes career of hers at She is also passionate about animals, nature and entrepreneurship.
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