Baby Development During the First Trimester

When you first find out you are pregnant, you want to know all about the changes that will happen as baby grows and develops. You are asking yourself what to expect and how to take care of your new tiny one. The first trimester is full of changes and by the end of the first trimester weeks (weeks 1-13), you will be growing a fetus that is developing daily.

By Chelsea Skaggs, founder of Postpartum Together

How Long is the First Trimester?

The average length of pregnancy is 40 weeks. The dates are calculated by adding 40 weeks to the date of your last menstrual period. The first trimester is 13 weeks long or the first 3 months of your pregnancy. You may not realize you are in your first trimester of pregnancy until a few weeks in when your body starts to change and send you signals. These early signs of pregnancy are from the hormone shifts that start to take place, signaling your ovaries to halt your menstrual cycle and produce estrogen and progesterone to support your new implantation.

  • Q: When does the first trimester end?
  • A: The first trimester ends at week 13 of pregnancy.
  • Q: What is the length of the first trimester?
  • A: The first trimester is 13 weeks long, though you may not know you are pregnant that entire time.
  • Q: Will I know I am pregnant in the first trimester?
  • A: Most women find out they are pregnant during the first trimester. This can be because of symptoms, missed period or another indicator.

Baby Changes in the 1st Trimester

In the first three months of your pregnancy, the systems and organs of your baby start to develop. During this time, many of your baby’s features are starting to form and the fetus goes from looking similar to a tadpole to looking like a baby.

  • Q: When does the baby have a heartbeat?
  • A: A heartbeat can usually be detected between 6-7 weeks gestation (American Pregnancy).
  • Q: What protects baby inside the womb?
  • A: The baby gets nourishment from the placenta, which grows with the baby. The amniotic fluid in the amniotic sac protects the baby.
  • Q: When does the fetus start to look like a baby?
  • A: At the end of the second month, your fetus takes on a baby form (and does not look like a tadpole anymore!)
  • Q: When do chromosomal abnormalities occur?
  • A: Chromosomal abnormalities usually occur during cell division, very early in pregnancy.
  • Q: When will I feel pregnant?
  • A: While everyone has a different reaction, many women notice their pregnancy in the second month. This is when hormones start to fluctuate and cause pregnancy symptoms.

The First Month of Pregnancy

When the sperm and egg first meet in your fallopian tubes, a zygote is formed. Typically, there are 46 chromosomes forming your baby at this time. Gender and chromosomal traits are determined, though they cannot be seen or detected yet. As the zygote moves from your fallopian tubes to your uterus, it implants into the uterine lining and becomes an embryo and the placenta. This is also the development of the amniotic fluid, which keeps your baby protected as it grows inside of you.

In the next couple of weeks, after implantation, the major systems and organs begin to form even though your embryo still looks like a tadpole. The neural tube, which eventually becomes the brain and spinal cord, begins to form along with the digestive system and circulatory system. In this first month, small buds start to form, which will become limbs.

The Second Month of Pregnancy

The second month is an exciting time for your growing fetus. The circulatory and nervous systems continue to develop, and the urinary and digestive systems are forming. The eye and ear structures become more recognizable and the buds that have formed look more like paddles with webbed fingers and toes. More features are recognizable during these weeks such as the upper lip, distinct head and nose. The bones are also forming.

This is the month when many women start to experience the rise of hormones, which can lead to headaches, morning sickness, and other signs of pregnancy. During this month, the embryo becomes known as a fetus and by the end of month two, all organs are developed, and baby’s heartbeat is detectable. The fetus is also gaining muscle and starting to move, though you will not yet feel the movement.

The Third Month of Pregnancy

During the third month of pregnancy, the fetus will begin to move more, though you likely won’t start to feel this movement until the second trimester. Your baby’s eyelids are now developed as well as fingernails and toenails. External genitals are formed during the third month as well as the larynx (voice box.) There are now tooth buds, which indicate where teeth will eventually grow after birth. While you will not hear baby yet, that means your baby will be prepared for crying in your future! With more recognizable arms and legs, baby is continuing to move in the amniotic fluid and develop those muscles. By the end of the third trimester, the fetus is an average of 3 inches long and weights around 1 ounce. It’s amazing that all of this detail is already formed in such a small fetus!

What to Do When You are in the First Trimester of Pregnancy

  • Q: When should I start taking prenatals?
  • A: It is beneficial to start prenatal vitamins as soon as you realize you are pregnant. Talk with your provider about the right vitamins for you.
  • Q: When should I schedule my first pregnancy appointment?
  • A: Contact your provider once you discover you are expecting. They will guide you to when they want to schedule your first appointment. This will likely be in the second month of pregnancy unless there is reason to have it sooner.
  • Q: Should I tell people I am pregnant once I find out?
  • A: This is a very personal choice. Decide how you want others to be involved in your journey and when you want to discuss your pregnancy with them.

Symptoms of the First Trimester of Pregnancy

The early symptoms of pregnancy are different for every woman. The most common first trimester symptoms women experience are:

  • Sore Breasts -Fatigue -“Morning sickness” (although this can happen anytime of the day!)
  • Implantation bleeding
  • Increased urination
  • Mood swings
  • Cravings
  • Food aversion -Headaches
  • Constipation -Heartburn

Staying Healthy in the First Trimester

When you discover you are pregnant in the first trimester, it is important to make changes to support your growing fetus. During this time American College of Obstetricians and Gynecologists recommends that mothers stop any smoking, alcohol consumption, drug usage, and avoid X-Rays. You need to choose a doctor or midwife and schedule your first appointment.

Healthy Diet

The first trimester is a great time to prioritize a healthy diet. There are supplements and foods you want to add in for baby’s development and foods to avoid during pregnancy.

What to eat during pregnancy:

  • Lean meats
  • Fibers
  • Fruits
  • Vegetables It is also a great time to start taking prenatal vitamins and folic acid to support your developing baby.

What to avoid during pregnancy:

  • Raw fish -High mercury fish
  • Unpasteurized dairy -Uncooked deli meat
  • High-levels of caffeine (1-2 cups if fine!)
  • Cat Litter

During these first weeks of your pregnancy, enjoy learning about all the changes that are happening. Schedule your first appointment with your OBGYN or midwife and take good care of yourself and your new little bundle! Learn more about all three trimesters as you and baby grow.

Postpartum Together founder Chelsea Skaggs

Chelsea Skaggs

Chelsea Skaggs is a postpartum advocate and coach who is committed to helping women kick the pressure to be “Pinterest Perfect” and have real, raw conversations to acknowledge and empower the postpartum experience. She provides small group coaching, eCourses, online communities and helps other women start motherhood-centered businesses. She believes that normalizing and empowering all the changes in life after baby can change the world and leads that effort at postpartumtogether.com.

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