Fetal Development in the First Trimester

Early Preparation: Weeks 1 and 2

During the first two weeks of the pregnancy timeline, the body prepares for conception, but pregnancy has not yet started. The menstrual cycle begins, and the uterine lining thickens to get ready for a fertilized egg. Ovulation typically happens near the end of this period. These weeks are included in the 40-week pregnancy calendar even though conception has not yet taken place.

Joining of Egg and Sperm: Week 3

Fertilization occurs around the third week. A sperm joins with an egg in one of the fallopian tubes, forming a single cell called a zygote. The zygote contains all the genetic material needed for the baby’s development, with 46 chromosomes—23 from each parent.

If more than one egg is released and fertilized, or if the zygote splits, twins or multiples can result. Soon after, the zygote divides many times, becoming a cluster of cells called a morula. This group of cells travels through the fallopian tube toward the uterus.

Attachment to the Uterus: Week 4

When the group of cells reaches the uterus, it becomes a blastocyst. The blastocyst embeds itself into the uterine lining, a process called implantation. Some cells inside the blastocyst become the embryo, while others start forming the placenta. The placenta provides nutrients and oxygen to the growing embryo.

Key Structures Developing

Structure	Function
Placenta	Nutrient/oxygen supply
Amniotic sac	Cushions embryo
Umbilical cord	Transport between mother/baby

Hormone Surges Support Growth: Week 5

About three weeks after conception, hormone levels rise quickly. The body produces more human chorionic gonadotropin (hCG), which tells the ovaries to stop releasing eggs and increases the production of estrogen and progesterone. These hormones keep the uterine lining thick and support the early stages of pregnancy. The embryo has three layers:

Ectoderm (outer): Becomes the skin, brain, spinal cord, eyes, and ears.
Mesoderm (middle): Becomes the heart, blood vessels, bones, muscles, kidneys, and most of the reproductive system.
Endoderm (inner): Becomes the lungs and intestines.

Notable Features

Early heart and simple blood vessels form.
Amniotic sac and placenta become more defined.

Shaping of the Brain and Spine: Week 6

In the sixth week, the neural tube, which will become the brain and spinal cord, closes along the back. This is an important step for the central nervous system. Other key events this week:

Heart forms distinct chambers and may start beating.
Early eyes and ears begin to develop.
Tiny limb buds appear, which will later become arms and legs.
The embryo’s shape curves into a C pattern.

Tip: Folic acid is especially important during this stage to help close the neural tube.

Forming the Head and Face: Week 7

During the seventh week, the embryo’s head grows quickly as the brain develops. Facial features start to appear as the depressions for nostrils and the beginnings of the eye’s retinas form. Other changes include:

The lower limb buds, which will be legs, become visible.
The upper limb buds start to look more paddle-shaped.

Quick List of Week 7 Developments

Depressions for nostrils
Start of retinas in eyes
Paddle-like arm buds
Leg buds appear

Nose, Ears, and Upper Lip: Week 8

Leg buds take on a more paddle-like shape. Fingers begin to form, though they are not fully separated yet. Swellings show where the outer ears will develop, and the eyes are now more noticeable. Other milestones:

Upper lip and nose form.
Trunk and neck begin to straighten.
Crown-to-rump length is about 1/2 inch (11–14 mm).

Important Terms

Facial features: Nose and upper lip visible.
External ears: Early swellings seen.
Limbs: Paddles with early finger development.

Appearance of Toes and Eyelids: Week 9

In the ninth week, arms grow longer and distinct elbows become noticeable. Toes are now visible on the feet. Eyelids develop and start covering the eyes. Other characteristics:

Head is still large compared to the rest of the body.
Chin is not yet well defined.
Length is just under 3/4 inch (16–18 mm).

Summary Table for Week 9

Feature	Status
Arms	Grow longer
Elbows	Visible
Toes	Appear
Eyelids	Form

Bending Elbows and Lengthening Limbs: Week 10

By the tenth week, the fetus develops more human-like features. The head becomes rounder, and the elbows can bend. The webbing between the toes and fingers disappears, making them look more separate and defined. Other developments:

Eyelids continue to cover the eyes.
Outer ears develop further.

Formation of Genitals and Baby Teeth: Week 11

By week eleven, the baby is now called a fetus instead of an embryo. Facial features grow more detailed, with a broad face, widely spaced eyes, and fused eyelids. The ears sit low on the sides of the head. In addition:

Buds for baby teeth form in the gums.
Red blood cells start developing in the liver.
The external genitals begin to form, though they are not distinct yet.
The fetus measures about 2 inches (50 mm) from crown to rump and weighs around 1/3 ounce (8 grams).

Development Checklist

Genitals: Visible as buds
Face: More defined
Blood cells: Form in liver
Teeth: Future baby teeth buds

Fingernails, Face Shape, and Organ Position: Week 12

By the twelfth week, fingernails start to grow. The face looks more typical, with a well-shaped profile. The intestines, which started developing outside the belly, now move into the abdomen.

Measurements

About 2.5 inches (61 mm) from top of head to tailbone.
Weighs around 1/2 ounce (14 grams).

Further Development

Fingernails are visible.
Fingers and toes look more separate and less webbed.
Eyes, ears, and eyelids become more distinct.
Organ systems continue to mature.

Week	Key Development	Size Estimate
4	Implantation, blastocyst forms	Microscopic
6	Neural tube closes, limb buds	Few millimeters
8	Facial features appear	1/2 inch (11–14 mm)
11	Genitals develop	2 inches (50 mm), 1/3 ounce
12	Fingernails, intestines move	2.5 inches (61 mm), 1/2 ounce

Key Body Systems and Structures in the First Trimester

Central nervous system: Brain and spine start forming from the neural tube.
Heart: Starts beating and circulates blood.
Placenta and umbilical cord: Support, nourish, and remove waste.
Limbs: Arms and legs, fingers and toes take shape.
Facial features: Eyes, ears, lips, and nose become more noticeable.