At six months of pregnancy (roughly weeks 24 through 27), a fetus is about 8 to 9 inches long from head to rump and weighs between 1.3 and 2 pounds. It looks unmistakably like a baby at this point, with fully formed facial features, visible fingers and toes, and skin that’s still thin and somewhat translucent. This is also the stage when the fetus crosses a major biological threshold: the earliest edge of viability outside the womb.
Size Week by Week
Growth is rapid during month six. At week 24, the fetus measures about 8.25 inches from crown to rump (not counting the legs) and weighs roughly 1.3 pounds. By week 26, it reaches about 9 inches and nearly 2 pounds. For a sense of scale, popular pregnancy references compare the 24-week fetus to the length of an ear of corn, growing to the size of a cauliflower head by week 27.
What the Face and Body Look Like
By six months, all the basic facial features are in place. The nose, lips, and ears are well defined. The eyelids, which were fused shut for months, begin to open around week 26 as the nerve pathway connecting the eyes to the brain finishes forming. Once the eyes open, the fetus can detect changes in light filtering through the uterine wall. Eyebrows and eyelashes are visible.
The body is lean, almost bony looking, because the fetus hasn’t yet built up the layer of fat it will accumulate in the third trimester. Fingers and toes are fully separated, and fingernails are growing. The skin appears reddish and wrinkled, partly because there’s not enough fat underneath to fill it out yet.
Skin, Hair, and Protective Coatings
Two distinctive features cover the fetus at this stage. The first is lanugo, a coat of soft, fine hair that develops between weeks 16 and 20 and is most visible on the back, shoulders, and tailbone, though it can grow anywhere there’s a hair follicle. It doesn’t appear on the lips, palms, or soles of the feet.
Lanugo exists to hold a second layer in place: vernix caseosa, a waxy, cheese-like coating that protects the skin from constant exposure to amniotic fluid. Without this combination, the amniotic fluid could damage the fetus’s delicate skin. The lanugo-vernix system also helps insulate the body and regulate temperature until enough body fat develops to take over that job. Most of the lanugo will shed before birth, though premature babies are sometimes born with patches of it still visible.
Brain Development and Senses
The sixth month is a period of explosive brain growth. Synapses (the connections between brain cells) are forming rapidly, and the brain is overproducing cells, dendrites, and neural connections in a genetically driven burst that will later be pruned and refined. Around week 24, a key milestone occurs: nerve fibers from the thalamus, which acts as the brain’s relay station, begin connecting to the cortex. This is what allows the fetus to start processing sensory input in a meaningful way.
Once those connections form, the fetus begins responding to pain signals, light, speech, and sound. Spontaneous brain activity patterns emerge that researchers have linked to more favorable brain and behavioral outcomes later in life. Sleep-wake cycles start to take shape, and you may notice that your baby’s movement patterns become more predictable, with active and quiet periods throughout the day.
Movement You Can Feel
By six months, fetal movements have progressed well beyond the early “butterfly kicks” of the second trimester. You’ll feel definite kicks, rolls, jabs, and nudges throughout the day. You may also notice rhythmic twitching that feels more like muscle spasms or pulsing. These are likely fetal hiccups, which are common and normal at this stage. The fetus is also stretching, grasping, and turning inside the uterus. As you move into the third trimester (which begins around week 28), your provider may ask you to start counting fetal movements, tracking how long it takes to feel 10 kicks, jabs, or pokes.
Lung Development and Viability
The lungs are one of the last organs to mature, and at six months they’re still in a critical building phase. Between weeks 16 and 26, the lung tissue is forming the tiny air sacs and gas exchange regions that will eventually allow breathing. Specialized cells in the lungs begin producing surfactant, a slippery substance that keeps the air sacs from collapsing, starting around week 24. But the lungs won’t be clinically mature until about week 35, which is why extreme prematurity is so dangerous.
Still, week 24 marks the generally accepted threshold of viability. Survival rates at 24 weeks vary widely depending on the hospital and the circumstances, but a commonly cited guideline places survival around 40% for babies born at 24 weeks and about 60% at 26 weeks. In hospitals with advanced neonatal care, those numbers can be higher. Babies born this early face significant medical challenges and typically spend months in intensive care, but survival is possible in a way it wasn’t just a generation ago.
What’s Happening on the Inside
Beyond what’s visible, several internal systems are maturing simultaneously. The inner ear is developed enough for the fetus to respond to sounds and possibly even recognize the rhythm of a parent’s voice. The digestive system is practicing by swallowing amniotic fluid. The bone marrow has taken over blood cell production. And the fetus is beginning to accumulate the brown fat that will help regulate body temperature after birth, slowly filling out the wrinkled skin and shifting from a translucent, angular appearance toward the rounder look of a full-term newborn.