Fetal Development Week by Week: A Complete Guide for Intended Parents

Overview

Pregnancy is measured in gestational weeks, counted from the first day of the last menstrual period, not the actual date of conception. Alongside gestational age, embryologists also track embryonic age, which starts two weeks later, at fertilization. For intended parents following a surrogate’s journey, understanding both timelines makes every ultrasound update and medical milestone far easier to follow.

Fetal Development Week by Week

First Trimester (Weeks 1 to 13)

Gestational Weeks 1–2 (0.2–0.5 Months)
These early weeks are counted before conception actually occurs, based on the start of the mother’s last menstrual cycle. Ovulation typically happens around week 2, setting the stage for fertilization.

Gestational Week 3 (0.7 Months) — Embryonic Week 1: Fertilization
Fertilization takes place, and the fertilized egg begins dividing rapidly as it travels toward the uterus. Implantation into the uterine lining usually occurs about a week later.

Gestational Week 4 (0.9 Months) — Embryonic Week 2: Implantation
The embryo implants into the uterine wall. Hormone levels rise enough for a pregnancy test to detect the pregnancy. The embryo is still smaller than a poppy seed.

Gestational Week 5 (1.2 Months) — Embryonic Week 3
The neural tube, which will become the brain and spinal cord, begins forming. The heart starts as a simple tube and will soon begin beating.

Gestational Week 6 (1.4 Months) — Embryonic Week 4
Cardiac activity can often be detected on ultrasound around this stage. Tiny buds that will become arms and legs start to appear.

Gestational Week 7 (1.6 Months) — Embryonic Week 5
The embryo is roughly the size of a blueberry. Hands and feet are forming as paddle-shaped structures, and the brain is developing rapidly.

Gestational Week 8 (1.8 Months) — Embryonic Week 6
All major organs have begun to form in a basic structure. Fingers and toes start to separate, and facial features become more defined.

Gestational Week 9 (2.1 Months) — Embryonic Week 7
Essential organs continue maturing. The tail-like structure at the end of the spine disappears, and the embryo’s proportions start to look more human.

Gestational Week 10 (2.3 Months) — Embryonic Week 8
This marks the end of the embryonic period. Every major organ system exists in basic form, and fingernails and toenails begin to develop.

Gestational Week 11 (2.5 Months) — Fetal Week 9
This is the point where the developing baby is medically referred to as a fetus rather than an embryo. Bones begin hardening, and tiny tooth buds form beneath the gums.

Gestational Week 12 (2.8 Months) — Fetal Week 10
Reflexes are developing, including the ability to open and close fingers. The risk of miscarriage drops significantly by the end of this week.

Gestational Week 13 (3.0 Months) — Fetal Week 11: End of First Trimester
Fingerprints have formed, and the fetus can make sucking motions. Vocal cords begin to develop.

Second Trimester (Weeks 14 to 27)

Gestational Week 14 (3.2 Months)
The fetus can make facial expressions and may start sucking its thumb. Fine hair called lanugo begins covering the body.

Gestational Week 15 (3.4 Months)
Taste buds are forming, and the fetus’s skeleton continues converting from cartilage to bone.

Gestational Week 16 (3.7 Months)
Some pregnant people may begin noticing early fetal movements, often called quickening, around this stage, while others may not feel clear movement until approximately 18 to 20 weeks or later.

Gestational Week 17 (3.9 Months)
The fetus is developing a layer of protective fat and continues gaining strength in its movements.

Gestational Week 18 (4.1 Months)
Hearing begins to develop, and the fetus may start responding to sounds from outside the womb.

Gestational Week 19 (4.4 Months)
A protective, waxy coating called vernix caseosa starts forming on the skin to guard against amniotic fluid exposure.

Gestational Week 20 (4.6 Months)
This marks the halfway point of pregnancy. Movements become stronger and more noticeable to the mother.

Gestational Week 21 (4.8 Months)
The fetus develops a more regular pattern of waking and sleeping, and swallowing motions become more coordinated.

Gestational Week 22 (5.1 Months)
Facial features are well defined, including eyebrows and eyelids. This is close to the early edge of the viability week range, though survival outside the womb remains uncertain.

Gestational Week 23 (5.3 Months)
The fetus’s inner ear is fully developed, helping with balance. Blood vessels in the lungs are developing in preparation for breathing.

Gestational Week 24 (5.5 Months)
This is widely considered the 24 weeks viability milestone. Lung development has progressed enough that, with intensive neonatal support, survival becomes possible, though extensive medical care would still be needed.

Gestational Week 25 (5.8 Months)
The fetus is gaining fat under the skin, and capillaries are forming to support developing lungs.

Gestational Week 26 (6.0 Months)
The eyes begin to open, and the fetus responds more clearly to light and sound.

Gestational Week 27 (6.2 Months): End of Second Trimester
Brain activity increases substantially, and the fetus has established more predictable sleep cycles.

Third Trimester (Weeks 28 to 40)

Gestational Week 28 (6.4 Months)
The fetus can blink and has regular sleep cycles. Survival rates for premature birth improve considerably compared to earlier weeks.

Gestational Week 29 (6.7 Months)
Rapid brain growth continues, and muscles and lungs keep maturing.

Gestational Week 30 (6.9 Months)
Bones are fully formed but still soft and flexible. The fetus is gaining weight steadily.

Gestational Week 31 (7.1 Months)
The fetus is beginning to fill more of the available space in the uterus, and movements may feel different as room becomes limited.

Gestational Week 32 (7.4 Months)
Most babies born at this stage survive with appropriate NICU support. Toenails and fingernails are fully developed.

Gestational Week 33 (7.6 Months)
The fetus practices breathing motions, and the skull remains soft and flexible to ease delivery.

Gestational Week 34 (7.8 Months)
The central nervous system and lungs continue maturing steadily.

Gestational Week 35 (8.1 Months)
Babies born now are considered late preterm and generally do well with monitoring.

Gestational Week 36 (8.3 Months)
The fetus is considered early term. Layers of fat continue building under the skin, helping with temperature regulation after birth.

Gestational Week 37 (8.5 Months)
Most organ systems, including the lungs, are ready to function independently.

Gestational Week 38 (8.7 Months)
The baby is considered full term. Final preparations for birth are underway.

Gestational Week 39 (9.0 Months)
The baby often drops lower into the pelvis in many pregnancies, a stage sometimes called lightening.

Gestational Week 40 and Beyond (9.2 Months)
This is the traditional due date, though a full-term range extends from 37 to 42 weeks. Every pregnancy reaches this final stage on its own timeline.

Critical Periods for Development by Organ System

Certain organ systems are more sensitive to disruption during specific windows of development. This table outlines when major structural formation occurs for each system, based on standard embryology references.

Structure / System	Critical Period (Embryonic Age, Weeks)	Equivalent Gestational Age (Weeks)	Later Notes
Central Nervous System	3–16	5–18	Functional brain development continues from 16 weeks embryonic age through term
Heart	3–6	5–8	Highest risk window for major structural heart formation
Eyes	4–8	6–10	Structural and functional development extends to term
Ears	4–9	6–11	Later development affects hearing function through 18 weeks gestational age
Limbs	4–5	6–7	Narrow, high-sensitivity window for arm and leg formation
Upper Lip	5–6	7–8	Key window for lip formation
Palate	6–9	8–11	Key window for palate closure
Teeth	6–8	8–10	Early tooth bud formation, continues developing later in pregnancy
External Genitalia	7–9	9–11	Sex differentiation occurs during this window
Urinary Tract	4–8	6–10	Major structural formation of kidneys, ureters, and bladder

Understanding Viable Pregnancy Weeks

Throughout this timeline, viable pregnancy weeks become a meaningful reference point for intended parents, particularly from 22 weeks onward. What is viability week in practical terms comes down to the point where survival outside the womb becomes medically possible, and this threshold continues improving with every week of continued pregnancy.

Every stage of fetal development brings its own milestones, and having a physician-led team to explain them clearly can make a surrogacy journey far less uncertain. Surrogacy4All’s medical team is available to answer questions at any point in your journey. Contact Surrogacy4All at (212) 661-7673, 1148 5th Ave 1C, New York, NY 10128.