First Trimester of Pregnancy: Baby’s Development

Pregnancy is such a beautiful and intricate part of life. It’s very exciting and is a time of many changes for the mother-to-be. Pregnancy is broken down into three developmental stages, called trimesters. Each trimester brings its own unique changes and symptoms, but today we will focus on 1st trimester changes.

Baby’s Progress in First Trimester weeks

While most women won’t have symptoms of pregnancy in the first two weeks, their bodies are already beginning to work overtime. The first trimester lasts from weeks 1 through 12, or months one through three. Let’s take a closer look at each week individually so you know what to expect in terms of your baby’s development:

Weeks 1 through 4

A pregnancy is calculated using the first day of your last menstrual period. Around 30 hours after conception, the egg will divide itself into two. This is referred to as a zygote. The cell will keep dividing and multiplying until the zygote moves down the fallopian tube and enters the uterus.

By the time you are one month pregnant, the egg has implanted itself into your uterus. At this point the “baby” is called a blastocyst. It’s very small only measuring about 0.2 mm in width, and containing around 200 cells.

The blastocyst consists of different cell layers. The ectoderm is the outer layer that will serve as the baby’s brain and nervous system. The mesoderm is the middle layer that will grow into the baby’s bones, heart, muscles, and blood vessels. Finally, the endoderm or inner layer will become the digestive and breathing systems. On the outside of the blastocyst are chorionic villi which are little tentacles that will become the placenta.

Week 5

During this week of pregnancy the baby’s organs will begin to develop, but it still looks more like a raspberry than a human.

Week 6

This is a busy week for your baby. Your baby’s neural tube which is located on the back will begin to close. Some basic features will also begin to form.

Week 7

The baby’s head and face will begin to develop this week. Your baby’s brain, eye lenses, and nostrils will also begin to form.

Stages of pregnancy
Stages of pregnancy

Week 8

Week 8 brings growth to the baby’s arms and legs, as well as fingers.

Week 9

During the ninth week of pregnancy, the baby will continue to gain strength. Other changes include further development to the facial features and arms.

Week 10

At this point the baby’s neck and head will begin to develop. The head will take on a rounder shape and the eyelids will begin to close.

Week 11

Your baby is officially a FETUS! By the eleventh week, the baby’s head is about half of the fetus’ total length, but the body will begin playing catch up this week. There are lots of changes during this week of pregnancy.

Week 12

You’ve reached the last week of the first trimester pregnancy! Two big events happen at this stage of pregnancy – fingernails begin to grow and the baby’s face finally has a human profile!

If you are wondering what to expect during the second trimester, check out our article “Pregnancy Guide – A Week by Week Journey For Every Mom To Be”. It will show you lots of information that begins with the first trimester and goes all the way through to the third trimester.

Trimester Symptoms – Changes to Your Body

It’s no secret that your body will change drastically during your pregnancy. Just as each trimester brings changes for your baby, the same goes for your body. Let’s take a look at some early pregnancy signs and trimester symptoms:

  • The absence of your period
  • Mild bleeding or spotting between your periods
  • White vaginal secretion
  • Dizziness
  • Food cravings or aversions
  • Frequent urination
  • Morning sickness from increased amounts of pregnancy hormones
  • Changes to your breasts or back pain
  • Mood changes or extreme fatigue
  • Gastrointestinal problems such as constipation

These are some of the most common symptoms of pregnancy in the first month. However, keep in mind that some women may only experience a few of them, while others may not have any. Regardless, first trimester symptoms can stop a woman in her tracks, especially during her first pregnancy. So, be sure to get plenty of rest and eat right to keep your energy levels up.

First Trimester Warning Symptoms To Pay Attention To

While it’s normal to have a wide range of pregnancy symptoms in the first trimester, there are also some things you should watch out for within the first 13 weeks. Here they are:

  • A constant thirst for water that can’t be quenched
  • Severe dizziness or other problems with your vision
  • Muscle aches, chills, and a fever over 101F
  • Quick weight gains or no weight gains
  • Heavy bleeding from the vagina
  • Painful urination
  • Unbearable abdominal pain

If you experience any of these symptoms, contact your health care provider immediately. One or more of these symptoms can be signs of a miscarriage.

As you can see, the first trimester of pregnancy is a busy time for both the baby and the mother-to-be. This can be one of the most tiring stages of pregnancy, so it’s best to take it as easy as possible, get plenty of rest, and eat a nutritious diet. Doing so will help you enjoy being pregnant rather than staying miserable suffering from its symptoms.

Original infographic via in 2016, content updated November 2020.

First Trimester of Pregnancy