This first trimester guide is part of our complete fetal development week-by-week series, following development from conception through birth. During just the first thirteen weeks of pregnancy, a single fertilized egg develops into a fetus with a beating heart, a growing brain, developing organs, tiny fingers and toes, and recognizable facial features.

Although the baby is still very small, nearly every major organ system begins forming during the first trimester. Understanding these early stages of development provides a fascinating look at how quickly pregnancy progresses from conception through the end of week thirteen.

First Trimester Timeline:

Weeks 1–2: Preparing for Pregnancy

Although pregnancy is measured from the first day of the last menstrual period, fertilization has not yet occurred. During these first two weeks, the body prepares for ovulation and the possible release of an egg.

Highlights

Pregnancy dating begins
Ovulation approaches
The uterus prepares for implantation

Week 3: Fertilization

A sperm and egg unite to form a single cell called a zygote. This cell contains a complete set of unique DNA and immediately begins dividing as it travels toward the uterus.

Highlights

Fertilization occurs
Unique genetic blueprint is established
Rapid cell division begins

Amazing Fact: Every person’s unique DNA is established at fertilization.

Week 4: Implantation

The developing blastocyst implants into the lining of the uterus. Structures that will become the placenta, umbilical cord, and amniotic sac begin forming, providing nourishment and protection throughout pregnancy.

Highlights

Implantation
Placenta begins developing
Pregnancy hormones increase

Weeks 5–6: Heart and Brain Development

The neural tube develops into the brain and spinal cord. Around this time, cardiac tissue begins rhythmic contractions, and healthcare providers may detect early cardiac activity on ultrasound around six weeks. Small buds that will become the arms and legs also begin to appear.

Highlights

Brain and spinal cord develop
Cardiac activity begins
Arm and leg buds appear
Eyes and ears begin forming

Did You Know? By the end of week six, the embryo is only about the size of a pea.

Weeks 7–8: Major Organs Take Shape

By the end of the eighth week, nearly every major organ system has begun developing. Bones begin replacing cartilage, facial features become more defined, and the embryo starts looking increasingly human. At the end of week eight, the embryo is officially called a fetus.

Highlights

Major organs developing
Bones begin forming
Facial features become recognizable
Embryo becomes a fetus

Weeks 9–10: Rapid Growth

Growth accelerates. Fingers and toes are fully separated, fingernails begin developing, muscles strengthen, and the baby can make small movements that are usually too subtle for the mother to feel.

Highlights

Fingers and toes fully formed
Fingernails begin growing
Muscles strengthen
External ears continue developing

Weeks 11–13: The Foundation Is Set

By the end of the first trimester, the fetus has developed recognizable facial features, functioning organ systems that continue maturing, and increasingly coordinated movements. Although still tiny, the foundation for the rest of pregnancy has been established.

Highlights

Facial features become distinct
Bones continue hardening
Hands can open and close
Organs continue growing and maturing

Continue Exploring Fetal Development

The first trimester lays the foundation for every stage that follows. As pregnancy continues, the baby grows rapidly, begins responding to sounds, gains weight, and prepares for life outside the womb.

These milestones — a beating heart at six weeks, fingers forming at eight, a baby who can hiccup at ten — aren’t abstract. They’re real, and at Caring Network, we believe every woman deserves to witness them firsthand.

Next: Second Trimester Fetal Development: Week-by-Week Growth Guide

Second trimester fetal development during pregnancy weeks 14–27