Fetal Development Understanding How Your Baby Grows

How Fetal Development Begins

Pregnancy officially begins when a fertilized egg implants into the uterine lining. During these first weeks, development happens quickly — even before many people realize they are pregnant.

Soon after implantation:

• Cells rapidly divide
• The placenta begins forming
• Hormones increase to support pregnancy
• Early organ systems begin developing

Explore what happens in the earliest days of fetal development

First Trimester Development (Weeks 1–12)

The first trimester is a time of rapid foundational growth. By the end of the first trimester, all major organ systems are present, though they continue maturing. This trimester often brings physical symptoms such as fatigue and nausea as your body adjusts hormonally.

During this stage:

• The neural tube forms
• The heart begins beating
• Major organs begin developing
• Limbs start to form
• Facial features begin taking shape

See a detailed week-by-week breakdown of first trimester development

Second Trimester Development (Weeks 13–27)

The second trimester is often described as a period of steady growth. Many parents first feel fetal movement during this trimester. This stage often brings a sense of reassurance as growth becomes more visible through ultrasounds and movement.

During this stage:

• Bones harden
• Muscles strengthen
• Skin develops
• Hearing becomes functional
• Movement becomes noticeable

Learn what to expect when you feel your baby move for the first time

Explore detailed second trimester development milestones

Third Trimester Development (Weeks 28–40)

The third trimester focuses on growth and preparation for life outside the womb. By full term, your baby is preparing for birth.

During this stage:

• Brain development accelerates
• Lungs mature
• Body fat increases
• Sleep-wake cycles develop
• Reflexes strengthen

Follow your baby’s growth and preparation for birth in the third trimester

Baby Development Month by Month

Some individuals prefer understanding pregnancy in months rather than weeks. This format may help visualize broader growth patterns.

Track your baby’s development month by month throughout pregnancy

Counting Baby Kicks

In later pregnancy, monitoring fetal movement can provide reassurance about well-being. Your healthcare provider may recommend tracking kick counts in the third trimester.

Learn how to count kick counts and what to look out for

Bonding During Pregnancy

Connection can begin long before birth. Bonding can deepen as you learn more about your baby’s development.

Some ways parents bond during pregnancy include:

• Talking or singing to your baby
• Gently touching your belly
• Attending prenatal appointments
• Journaling about your experience

Discover 16 meaningful ways to bond with your baby during pregnancy

Gestational Development and Milestones

Gestational age is typically measured from the first day of your last menstrual period. Ultrasounds may help estimate growth and confirm dating.

Explore gestational development milestones and how age is calculated

When Should I Tell Others?

Many parents wonder when to share pregnancy news. While there is no single right time, some choose to wait until after the first trimester when miscarriage risk decreases. Sharing pregnancy news is a personal decision.

Explore the considerations around when to announce your pregnancy

Every Pregnancy Is Unique

Development follows predictable stages, but every pregnancy is unique. Understanding development helps you feel more informed and connected as your baby grows.

Factors that may influence fetal development include:

• Maternal health
• Nutrition
• Prenatal care
• Genetics
• Environmental exposures

Get guidance on nutrition and health habits that support fetal development