At five months pregnant, which corresponds roughly to weeks 17 through 20 of gestation, a baby typically weighs between 5 and 10 ounces (140 to 300 grams). By the end of the fifth month, at the 20-week halfway mark, most babies weigh close to 10 ounces and measure about 6 to 7 inches from crown to rump, or about 10 inches from head to toe.
What “Five Months Pregnant” Means in Weeks
Pregnancy months don’t map neatly onto calendar months, which is why doctors track progress in weeks instead. The fifth month generally covers weeks 17 through 20. Week 20 is considered the halfway point of pregnancy, and it’s when most people have their detailed anatomy scan ultrasound. That scan gives your care team its first solid estimate of how much your baby weighs.
Week-by-Week Weight During Month Five
Babies grow rapidly during the second trimester, so the difference between the beginning and end of month five is significant:
- Week 17: approximately 5 ounces (140 grams), roughly the weight of a small apple
- Week 18: approximately 7 ounces (190 grams)
- Week 19: approximately 8.5 ounces (240 grams)
- Week 20: approximately 10 ounces (280 to 300 grams), close to the weight of a can of soda
These are averages based on standard growth charts. A healthy baby can fall somewhat above or below these numbers and still be developing perfectly normally. The overall growth trend matters more than any single measurement.
What’s Developing at This Stage
The fifth month is a busy period for development beyond just weight gain. Around week 20, many parents feel their baby’s movements for the first time, sometimes described as flutters or light taps. The baby’s skin is developing a waxy protective coating, fine hair covers the body, and the limbs are now proportional to the rest of the body. The nervous system is maturing quickly, and the baby is beginning to respond to sound and light.
This is also when the anatomy scan typically takes place. That ultrasound checks the baby’s organs, spine, brain, and heart, and it provides the estimated fetal weight that your provider uses to track growth going forward.
How Ultrasound Weight Estimates Work
Your baby’s weight at this stage isn’t measured directly. Instead, an ultrasound technician measures the head circumference, abdominal circumference, and thigh bone length, and software calculates an estimated weight from those numbers. This method is useful but imperfect. There is an accepted margin of error of up to 20% in either direction, meaning a baby estimated at 10 ounces could actually weigh anywhere from 8 to 12 ounces.
That margin is worth keeping in mind if your provider mentions that your baby is measuring slightly large or small. A single estimate at this stage is a snapshot, not a verdict. What matters more is whether the baby continues growing at a consistent rate over time.
What Affects Fetal Weight
Several factors influence how much a baby weighs at any point in pregnancy, and some are within your control while others aren’t.
Genetics
Parental size is one of the strongest predictors of fetal size. Taller parents tend to have larger babies, and smaller parents tend to have smaller babies. A baby who measures on the lower end of the growth chart because of genetic factors is not the same as a baby whose growth has slowed due to a problem. Genetic size differences are normal variations, not complications.
Maternal Nutrition
What you eat directly affects how well nutrients reach your baby. When energy and protein intake are insufficient, a chain of metabolic responses reduces the exchange of nutrients between mother and fetus. Deficiencies in specific nutrients, particularly zinc, iron, iodine, and folate, have been linked to restricted fetal growth. Maternal weight gain during the first and early second trimesters primarily reflects the growth of maternal tissues, while gains from late in the second trimester onward reflect fetal and placental growth more directly.
Placental Health
The placenta is the delivery system for everything your baby needs. Early maternal weight gain appears to reflect how well the placenta itself is growing and functioning. A well-developed placenta transports nutrients and growth-supporting hormones more effectively, which supports steady fetal weight gain through the second half of pregnancy.
When Fetal Weight Raises Concerns
Doctors define fetal growth restriction as an estimated weight below the 10th percentile for gestational age. At 20 weeks, that would mean a baby weighing noticeably less than the average range. The Society for Maternal-Fetal Medicine also uses an abdominal circumference below the 10th percentile as an additional indicator. Severe growth restriction is defined as an estimated weight below the 3rd percentile.
If your baby’s measurements fall in this range at the anatomy scan, your provider will likely schedule follow-up ultrasounds to watch the growth trend. A single low measurement doesn’t necessarily mean there’s a problem, especially given the margin of error in ultrasound estimates. What providers look for is a pattern: is the baby continuing to grow, even if slowly, or has growth stalled? Consistent tracking over several weeks gives a much clearer picture than one data point.
On the other end, a baby measuring above average at 20 weeks is rarely a concern. Most large-for-gestational-age issues are monitored more closely in the third trimester, when the baby’s size has practical implications for delivery planning.
What a Healthy Fifth Month Looks Like
If you’re at your 20-week appointment and your baby weighs around 10 ounces, that’s right in the middle of normal. If the number is a bit higher or lower, that’s also likely fine, particularly if you and your partner are on the taller or shorter side. The key signals your provider is watching for are steady growth between appointments, normal organ development on the anatomy scan, and adequate amniotic fluid levels. Weight is one piece of that puzzle, not the whole picture.