Baby Size by Week

The blastocyst completes implantation into the uterine lining. Cells begin early specialization into the embryo, amniotic sac, and placenta.

The neural tube develops along the embryo's back, which will form the brain and spinal cord. Primitive blood vessels and heart tube begin to form.

Small limb buds appear that will form arms and legs. The primitive heart tube shows early rhythmic contractions that can often be detected on ultrasound.

Arm and leg buds lengthen and paddle-shaped hand and foot plates start forming. The brain and head grow rapidly compared to the rest of the body.

Webbed fingers and toes begin to separate into distinct digits. The eyelids and external ear folds form, and early involuntary movements begin.

Arms bend at the elbows and hands meet near the chest. The embryonic tail regression is complete, marking the approaching transition to the fetal period.

The embryonic phase ends; the developing baby is now medically classified as a fetus. Vital organs including kidneys, intestines, and liver are functioning in early stages.

The head makes up roughly half the fetal length. Facial features continue to refine as nasal bones, tooth buds, and external ear canals become established.

The kidneys begin producing small amounts of urine that pass into the amniotic fluid. This is the optimal window for first-trimester ultrasound dating and nuchal translucency scans.

The final week of the first trimester. Unique epidermal ridges (fingerprints) form on tiny fingertips, and intestines move fully into the abdomen.

Second trimester begins. The neck elongates and the chin lifts off the chest. Facial muscles exercise through subtle grimacing and squinting reflexes.

Bones continue to harden, and hair patterning begins on the scalp. Fine, downy hair called lanugo starts appearing across the delicate skin.

Adipose fat tissue begins accumulating beneath the skin to support heat generation and energy storage. The umbilical cord thickens and strengthens.

Myelin insulation begins forming around nerve fibers in the spinal cord. The fetus regularly swallows amniotic fluid, practicing digestive tract movements.

A protective white waxy coating called vernix caseosa covers the skin to shield it from amniotic fluid. This is the final week using the Crown-Rump Length convention.

Midpoint of pregnancy (20 completed weeks). Measurement standard changes from Crown-Rump Length to full Head-to-Heel length. Detailed mid-pregnancy anatomy scans typically occur around this week.

Bone marrow assumes major red blood cell production from the liver and spleen. Fetal sleep-wake patterns become more discernible through activity rhythms.

Tactile touch and grip reflexes strengthen as the fetus explores the uterine wall and umbilical cord. Estimated fetal weight reaches approximately 525 g at the INTERGROWTH-21st 50th percentile.

Rapid eye movements (REM) emerge during rest cycles. Blood vessels within the lungs multiply in preparation for eventual respiratory function after birth.

Specialized cells in the lungs begin producing pulmonary surfactant, a compound essential for keeping air sacs open after delivery. The fetus responds to acoustic vibrations.

Capillary beds form beneath the skin, giving it a pinkish tone. The fetus exhibits startle reflex movements in response to sudden loud noises.

Eyelids that have been fused shut since the first trimester begin to open. Primitive air sac precursors continue developing in the lungs.

The final week of the second trimester. Fetal brain tissue grows rapidly with increasing cortical convolutions. Rhythmic hiccup movements may be felt as gentle twitches.

Third trimester begins (28 completed weeks). The fetus can blink and turn toward sustained light. Brain tissue expands and develops complex cortical convolutions.

Bones are fully developed but remain soft and pliable to facilitate delivery. The brain can regulate primitive body temperature rhythms.

Amniotic fluid volume reaches its peak and begins to gradually decline as the fetus occupies more uterine space. Red blood cells are now completely manufactured by bone marrow.

Pupillary light reflex develops; pupils can constrict and dilate in response to light. Subcutaneous white fat continues depositing under the skin.

Toenails are visible and fingernails reach the tips of the fingers. Movement sensations shift from large somersaults to targeted kicks and stretches due to confined space.

Maternal antibodies (primarily IgG) actively cross the placenta to provide temporary immune protection after birth. Skull bones remain soft and pliable with open sutures.

The central nervous system and lungs continue maturing rapidly. The protective vernix caseosa thickens in skin creases across the body.

Kidneys are fully mature and the liver can process waste products. Most physical organ development is complete; remaining weeks are dedicated to healthy weight gain.

The head may begin descending into the maternal pelvic inlet (lightening or engagement). Most of the lanugo coat has shed into the amniotic fluid.

Reaching 37 weeks is clinically classified by ACOG as 'Early Term' (37 0/7 to 38 6/7 weeks). Fetal organ systems are capable of functioning independently outside the womb.

Organ systems are fully ready for extrauterine life. Vernix caseosa continues to shed into the amniotic fluid while intestines accumulate meconium.

Reaching 39 weeks is medically designated as 'Full Term' (39 0/7 to 40 6/7 weeks), the optimal window for neonatal outcomes. The chest is prominent and fat reserves are well-established.

Reaches the official estimated due date (280 days from LMP). Only a small percentage of births occur on the exact due date; most healthy arrivals occur within two weeks before or after.