At 25 weeks of pregnancy, a baby typically weighs about 1¾ pounds (roughly 680 to 750 grams) and measures around 13¼ inches from head to heel. That’s about the size of an acorn squash. Keep in mind that “normal” covers a range, and individual babies can fall above or below this average while still developing perfectly well.
What 25 Weeks Looks Like
By this point in the second trimester, your baby has crossed a major threshold. At 25 weeks gestational age (23 weeks after conception), the baby responds to familiar sounds, including your voice. Most sleep time is spent in rapid eye movement (REM) sleep, the same phase of sleep associated with dreaming in adults. The eyelids are still closed, but the eyes move quickly beneath them.
The skin is still thin and somewhat translucent, but fat deposits are starting to fill in, which is why weight gain accelerates noticeably over the coming weeks. The lungs are developing the structures needed for breathing air, though they aren’t mature enough to function well on their own yet. Tiny blood vessels called capillaries are forming throughout the lungs, preparing for the eventual exchange of oxygen after birth.
How Ultrasound Weight Estimates Work
If your provider gave you a weight estimate from an ultrasound, it’s worth knowing that these numbers aren’t exact. Ultrasound calculates fetal weight using measurements of the baby’s head, abdomen, and thigh bone, then plugs those into a formula. The most commonly used formula (called Hadlock) is considered the most reliable, but even so, estimates can be off by a meaningful amount.
Research has found that most ultrasound weight formulas are accurate within a range of plus or minus about 500 grams (roughly one pound) at their best. Ultrasound also tends to overestimate weight, particularly in larger babies. So if your 25-week scan shows a weight slightly above or below the average, the true weight could easily be different. A single measurement that’s a bit high or low is rarely a concern on its own. Providers look at the trend over multiple visits to judge whether growth is on track.
What Your Body Is Doing at 25 Weeks
Your fundal height, the distance from your pubic bone to the top of your uterus, should measure close to the number of weeks you are in pregnancy, plus or minus 2 centimeters. So at 25 weeks, a measurement between 23 and 27 centimeters is considered normal. Your provider checks this at routine visits as a quick gauge of whether your baby’s growth matches expectations. A measurement that falls outside that window doesn’t automatically mean something is wrong, but it may prompt a closer look with ultrasound.
Factors That Affect Fetal Weight
Plenty of things influence how big or small a baby measures at any given point in pregnancy. Some are genetic and completely out of your control: taller parents tend to have longer babies, and a baby’s own chromosomal makeup plays a role. Carrying twins or triplets also means each baby will typically weigh less individually than a singleton at the same gestational age.
Maternal health conditions can slow fetal growth. High blood pressure, diabetes, anemia, chronic lung or kidney disease, and autoimmune conditions like lupus are all linked to a pattern called fetal growth restriction, where the baby measures significantly smaller than expected. Problems with the placenta or umbilical cord, such as poor attachment or limited blood flow, can also restrict the nutrients reaching the baby.
Lifestyle factors matter too. Cigarette smoking, alcohol use, drug use, poor nutrition, and very low maternal weight all increase the risk of growth restriction. On the other end, maternal obesity and poorly controlled diabetes can lead to a baby that’s larger than average. If your provider identifies a growth concern in either direction, they’ll monitor more frequently with additional ultrasounds to track the trajectory.
What If Baby Is Born at 25 Weeks
A baby born at 25 weeks is extremely premature but has crossed into the zone of viability. With intensive care, the survival rate for babies born at 25 to 26 weeks is around 80%. That’s a meaningful number, but it comes with significant medical challenges that require weeks or months in a neonatal intensive care unit (NICU).
Lung problems are the most common hurdle. About 1 in 3 babies born this early develop a chronic lung condition that requires extended breathing support. Brain complications affect roughly 1 in 10, and a similar proportion develop serious eye problems that may need laser treatment. Severe bowel infections occur in about 1 in 20.
Long-term outcomes vary widely. About 1 in 10 survivors have serious movement or learning challenges that require lifelong support, and a similar proportion develop cerebral palsy. Roughly 1 in 15 have a severe learning disability. Many others, however, go on to develop within a typical range, particularly with early intervention services. The gap between 25 weeks and full term (39 to 40 weeks) is enormous in terms of organ maturity, which is why every additional week in the womb at this stage meaningfully improves outcomes.