At every scan appointment, one of the most frequent questions asked by moms-to-be is, ‘Is my baby growing well? Your doctor cares a lot about how much weight you gain during your pregnancy. It tells him a lot about how your baby is growing, whether the placenta is doing its job, and if you need some extra attention before your baby is born.
This guide takes you through average fetal weight by trimester and week, explains how doctors measure it, covers what pushes weight up or down, and tells you when to ask your doctor a question.
How Doctors Measure Baby Weight in the Womb
You can’t directly measure your baby’s weight before birth.
Doctors estimate it using 4 key dimensions measured by ultrasound scans:
- Biparietal diameter (BPD): Head width
- Head circumference (HC): Head circumference around the head
- Abdominal Circumference (AC): Circumference of the belly
- Femur length (FL): Length of the thigh bone
These measurements are then inserted into formulas to provide an estimated fetal weight (EFW). These four biometric parameters have been confirmed as the international standard for fetal weight estimation by ISUOG, the International Society of Ultrasound in Obstetrics and Gynecology, which has produced practice guidelines.
A couple of things to keep in mind: Ultrasound weight pregnancy yoga classes estimates are not the exact science we’d like them to be—there’s a margin of error of roughly 10 to 15%. So see each number as a range, not a fixed number. Your doctor will record each measurement of your baby and compare it to growth charts based on the population to see if your baby is within the expected range for that gestational age.
Average Weight of Baby During Pregnancy: Week by Week
Below is a chart of generally accepted average fetal weight estimates. These figures are based on published obstetric data and WHO standard growth curves. All babies grow at their own pace, so a reading slightly above or below these averages is usually nothing to worry about. If this is something that needs to be looked at, your doctor will let you know.
First Trimester: Weeks 8 to 12
Then growth is measured in milligrams and grams. The mass is still very small, but the baby’s organs, limbs, and facial features are forming quickly.
| Week | Average Weight | Length (approx.) |
| 8 | 1–2 grams | 1.6 cm |
| 9 | 2–3 grams | 2.3 cm |
| 10 | 4–5 grams | 3.1 cm |
| 11 | 7–8 grams | 4.1 cm |
| 12 | 14 grams | 5.4 cm |
At the end of the first trimester, your baby weighs about as much as a large grape.
Second Trimester: Weeks 13 to 26
This is when the growth really kicks in. Bones grow stronger, muscles grow bigger, and fat begins to build up. You might start to feel movement around 18 to 20 weeks.
| Week | Average Weight | Length (approx.) |
| 13 | 23 grams | 7.4 cm |
| 14 | 43 grams | 8.7 cm |
| 16 | 100 grams | 11.6 cm |
| 18 | 190 grams | 14.2 cm |
| 20 | 300 grams | 25.6 cm |
| 22 | 430–478 grams | 27.8–29 cm |
| 24 | 600–670 grams | 30–32 cm |
| 26 | 760–913 grams | 35.1–35.6 cm |
By week 20, the baby is around the size of a small mango. By the 26th week, about the size of a small eggplant.
Third Trimester: Weeks 27 to 40
This is the fastest weight gain phase of the entire pregnancy. In the last few weeks, your baby will put on about 200 to 300 grams a week, laying down fat stores and maturing the lungs and brain.
| Week | Average Weight | Length (approx.) |
| 27 | 875 grams–1.05 kg | 36.6 cm |
| 28 | 1.0–1.2 kg | 37.6 cm |
| 30 | 1.3–1.5 kg | 40 cm |
| 32 | 1.7–1.8 kg | 42.4 cm |
| 33 | ~2.0 kg | 44.1 cm |
| 34 | ~2.2 kg | 45.3 cm |
| 36 | ~2.6 kg | 47.4 cm |
| 38 | ~3.0 kg | 49.8 cm |
| 40 | 3.3–3.6 kg | ~51 cm |
A full-term baby (40 weeks) will usually weigh between 2.8 and 3.5kg.
Low birth weight (LBW) is defined by WHO as a birth weight less than 2.5 kg regardless of gestational age.
Baby Weight by Trimester: A Simple Summary
If you’d like a quick reference without going week by week, here’s the trimester-level picture.
- First Trimester (Weeks 1 to 12) Weight at end of trimester: ~14 g. All main organs are forming, but the actual mass is negligible. Don’t worry about gaining weight at this stage.
- Second Trimester (Weeks 13-26) Weight at end of trimester: 760 grams to just under 1 kilogram. Why ripe papaya not as dangerous. This is where the growth curve begins to steepen. Routine anomaly scans at about 18 to 20 weeks check organ structure and growth.
- Third Trimester (Weeks 27-40): Average weight at end of trimester: 3.3-3.6 kg. Now the baby has the most weight. Each week, with each scan during this time, your doctor can best see how the baby is progressing toward birth.
What Is a Normal Birth Weight?
A full-term baby is born at 37 to 42 weeks and usually weighs between 2.5 kg and 4.0 kg. Anything above 4.0 to 4.5 kg is classified medically as macrosomia (large for gestational age), and anything below 2.5 kg fits the WHO’s definition of low birth weight (LBW).
It is good to know that Indian newborns tend to be a little smaller than the international average and is horse gram good for pregnancy. for Indian mothers. Birth weight is 2.5 to 4 kg and body length 48 to 52 cm.
What Affects Your Baby’s Weight During Pregnancy?
There are many factors that affect your baby’s growth and weight gain. This is what the study reports.
Maternal Nutrition
This is one of the more directly manipulable variables. A cross-sectional study in the National Library of Medicine revealed a direct correlation between birth weight and maternal nutritional and dietary factors. Mothers who consumed balanced diets with adequate protein, calories, and micronutrients were also much more likely to deliver babies within the healthy weight range. Low birth weight is associated with skipping meals, poor eating in the first 8 weeks, or an unbalanced diet.
These nutrients directly aid fetal weight gain: Protein (muscle and tissue). Iron (placental function and oxygen delivery). Calcium (bone density). Zinc (cell growth).
Genetics and Parental Build
A baby’s growth trajectory is determined by its genetic blueprint. Taller parents with bigger frames tend to have bigger babies. This is normal variation and is not a sign of any problem.
Placental Health
The placenta passes oxygen and nutrients from your blood to your baby. Growth restriction may be caused by decreased blood flow to the placenta resulting from hypertension, preeclampsia, or other conditions. Hypertensive disorders and placental dysfunction are some of the leading causes of fetal growth restriction (FGR) worldwide, as highlighted by the FIGO initiative on fetal growth in the International Journal of Gynecology and Obstetrics.
Gestational Diabetes
Well, if the mother has high blood sugar, it can pass through the placenta and make the baby produce more insulin, which acts as a growth hormone. This causes macrosomia (the largest average baby), which increases the risk of delivery complications.
Smoking, Alcohol, and Drug Use
The fact that cigarette smoking reduces birth weight by an average of 200 grams in a dose-response pattern is confirmed by research in the FIGO journal. Alcohol use is linked with fetal growth restriction and fetal alcohol spectrum disorders.
Multiple Pregnancies
Multiple Pregnancies: Twins, triplets, and higher-order multiples all get a smaller share of placental resources, so individual baby weights in multiple pregnancies are usually lower than in singleton pregnancies.
Stress and Mental Health
Chronic stress during pregnancy raises cortisol levels that can impact placental blood flow and fetal growth. This is a reason to take rest seriously, not as a soft suggestion.
When Should You Be Concerned About Baby’s Weight?
Fewer. Ultrasound weights are estimates, not exact figures. Usually, a single reading outside the average range does not mean anything is wrong. “Doctors are looking for a pattern.
If you speak to your doctor:
- Consecutive scans show the baby’s growth percentile decreasing over time
- Estimated fetal weight <10th percentile for gestational age (this is the clinical threshold for small for gestational age or SGA)
- Your baby’s movements are noticeably less after 28 weeks
- Your fundal height (distance from pubic bone to top of uterus) is much smaller than expected for your dates
Your doctor may suggest:
- Growth scans at shorter intervals
- Doppler studies to evaluate blood flow through the umbilical cord
- Modified bed rest or modified diet
- Planning delivery earlier in some high-risk situations
Remember, ACOG is clear that the only way to truly confirm birth weight is to weigh a newborn after delivery. The prenatal estimates are useful for monitoring and decision-making, but they are not precise measurements.
How to Support Healthy Baby Weight Gain
You are more in control than you think. These are the steps that make a quantifiable difference: Get sufficient protein. Protein is part of all the tissues in your baby’s body. From the second trimester, aim for 70–100 grams a day. Good sources include dal, paneer, cooked eggs, chicken, and Greek yogurt.
- Don’t skip your prenatal checkups. Growth scans at the anomaly scan (weeks 18-20) and at weeks 28, 32, and 36 provide your doctor with the information required to identify any growth deviation early.
- Exercise safely. Prenatal yoga and walking support healthy placental blood flow, which directly feeds your baby. “Women with no contraindications should engage in aerobic and strength-conditioning activities during pregnancy to support healthy fetal development,” the NIH adds.
- Manage your blood pressure and blood sugar. Hypertension and gestational diabetes have a direct effect on fetal weight. Regular monitoring and diet modifications do make a difference.
- Don’t smoke or drink alcohol at all. The evidence is straightforward. Both reduce birth weight, and there are other risks to fetal brain development.
- Sleep on your left side from the second trimester onwards. Sleeping on your left side increases the flow of blood to the uterus and placenta, giving your baby better access to nutrients and oxygen.
Mom’s Preg-Ladder directly addresses these factors through nutritional counseling and early pregnancy classes customized for each trimester, helping moms develop habits that promote healthy fetal growth, week by week.
What Happens to Baby Weight in the Final Weeks?
The last four weeks of pregnancy are the most active period for weight gain. In the last month, babies generally gain 200 to 300 grams of weight a week. After birth, fat is laid down under the skin to help regulate temperature. The lungs are maturing, and the brain is still developing rapidly. The baby at 36 weeks weighs about 2.6 kg. By 38 weeks, approximately 3.0 kg. At 40 weeks, the average is 3.3 – 3.6 kg. Many mothers worry if, by the 38th week, their baby has not reached 3 kg. Let’s analyze it. Babies gain weight at different rates even in the last weeks, and a baby at the lower end of the normal range is not automatically in danger. What is important is not whether it hits a certain number on a certain date, but whether the growth is on a consistent curve.
Indian Mothers: A Note on Growth Charts
There are many growth charts available online based on Western population data. Indian babies are genetically born a little smaller than the world average, with normal birth weights of between 2.5 and 4 kg. If your baby is just below an international chart, but your doctor tells you that growth is good and the placenta looks healthy, there is probably nothing to worry about. Mom’s Preg-Ladder’s programs are geared towards Indian mothers, with emphasis on nutrition, prenatal movement, and birth preparation in the context of local food, lifestyle, and physiology.
5 Frequently Asked Questions
1. What is the normal weight of a baby during pregnancy at 7 months (28 weeks)?
At 28 weeks, the average fetus is between 1.0 and 1.2 kg in weight and 37.6 cm in length. This is when the baby’s eyes start to open, and the brain goes into a rapid growth phase. Weight gain accelerates from now on, and the baby gains about 200 to 300 grams each week in the last trimester.
2. What is considered a healthy baby weight at the time of birth?
Overall, a full-term baby (born between 37 and 42 weeks) is healthy with a birth weight between 2.5 kg and 4.0 kg. WHO defines low birth weight as less than 2.5 kg. Babies from 4.0 to 4.5 kg are considered large for gestational age, or macrosomic, and may need extra monitoring during delivery.
3. Can I increase my baby’s weight during pregnancy?
Yeah, to a meaningful degree. Protein, iron, calcium, and calories in adequate amounts directly promote fetal weight gain, especially in the second and third trimesters. There are practical things you can do, like staying active, managing your blood sugar and blood pressure, sleeping on your left side, and attending all your growth scan appointments. Talk to your doctor or a certified pregnancy health coach if your scans show the baby tracking in ranges below expected.
4. How accurate is an ultrasound for measuring baby weight?
The margin of error for ultrasound-based estimated fetal weight is about 10 to 15% by obstetric research. That’s the reason why doctors analyze trends across several scans, not a single number as gospel. The accuracy is best in the first and second trimesters, and the estimate becomes less accurate as the baby grows larger in the third trimester.
5. Is it normal for my baby’s weight to seem small at 32 weeks?
A baby being a little below average on one scan at 32 weeks does not mean that there is a problem. What doctors look for is a pattern of decreasing growth percentiles on subsequent scans, or a weight below the 10th percentile for gestational age. A single low reading is generally not a concern if your doctor is satisfied with the Doppler blood flow results and your placenta appears to be in good condition. Always talk to your obstetrician about any concerns.
