What should be the weight of the fetus by weeks of pregnancy?

An essential component of pregnancy is comprehending fetal development. A baby’s weight is a major indicator of health, and it varies every week as the child grows. Fetal growth milestones vary depending on the stage of pregnancy, so being aware of what to anticipate can reassure you and keep you informed.

Fetal weight rises quickly during pregnancy, indicating the baby’s growth and development. Monitoring this growth helps make sure that everything is developing normally, from the first few weeks when the fetus is just a tiny cluster of cells to the later stages when it becomes a fully formed baby.

What a normal fetal weight should be at each stage of pregnancy will be discussed in this article. We’ll break it down week by week so you can see the progression of your baby and anticipate changes as the months pass.

Week of Pregnancy	Average Fetal Weight
4 weeks	Less than 1 gram
8 weeks	About 1 gram
12 weeks	About 14 grams
16 weeks	About 100 grams
20 weeks	About 300 grams
24 weeks	About 600 grams
28 weeks	About 1 kilogram
32 weeks	About 1.7 kilograms
36 weeks	About 2.5 kilograms
40 weeks	About 3.4 kilograms

Recognizing the fetal weight at each week of pregnancy aids in tracking the growth and development of your unborn child. Following these changes from the beginning to the end can give comfort and point out any possible problems. You can make sure your baby is on track for a healthy delivery and better prepare for routine check-ups by being aware of the average weight range for each week.

How the baby grows?

All babies grow exactly the same at the start of pregnancy, regardless of gender, race, or genetics; they add grams and millimeters to their tiny weight every week. Every embryo develops roughly at the same rate; differences only show up later, when the distinct features of the tiny person start to take shape.

Only in the latter part of the first trimester—and even with the most advanced ultrasound machine—is it feasible to gauge the fetus’s growth. The only criterion that is measured up to eight weeks is the ovum’s diameter, which is thought to be the primary indicator of the embryo’s growth.

The crown-rump size is the only measurement that is currently available for the baby, but it is not sufficient to determine the weight of the embryo.

Beginning in the eleventh week of pregnancy, it is possible to discuss the fetus’s weight for the first time during an ultrasound examination. At this point, the first variations in baby size start to show. Some babies are now larger than others. The baby weighs about 2 grams at 7-8 weeks, but this is so little that the weight is not useful for diagnosis.

The baby starts to actively gain weight after 12 weeks, sometimes doubling its weight in a single week. All systems and organs are typically formed at this point, and a period of rapid development and enhancement starts. The baby gradually starts storing subcutaneous fat, which explains the quick weight gain. The baby in the third trimester stands to gain the most.

The baby has spent the last few months becoming physiologically ready for delivery; all of its organs and systems are functioning, and it just needs to "get better" a little. The baby, who has grown significantly, can no longer move around actively in the uterus and now spends most of his time sleeping and occasionally making sharp movements in his constricted uterus. The baby puts on the weight he will have at birth during the last two months, and the weight gain only nearly stops in the final days before delivery.

Female fetuses gain slightly less than male babies starting at the end of the second trimester. Because of this, in the third trimester they are nearly always identified by smaller forms; however, this is not always the case as some girls have the ability to outweigh any boy in terms of weight.

What affects weight?

Prenatal weight is largely determined by heredity. The likelihood that a big, large mother and father will give birth to the same "hero" is greater than 90%. Babies born to petite, short, thin parents typically weigh little. Any outside factors finding their way into this pattern will find it extremely difficult.

Pregnant women should keep in mind that certain things will inevitably have an impact on the body weight of the unborn child. This is nutrition first and foremost. A mother who eats healthily and in moderation will provide her unborn child with a balanced diet that will support the child’s healthy development and growth. The child receives all of this through the uteroplacental blood flow if the expectant mother overeats, loves carbs, cakes, and pastries, and abuses vitamin complexes—even if she does not have a vitamin deficiency. As the mother’s weight increases, so does the baby’s.

If the mother’s diet is inadequate, lacking in proteins, carbs, and vitamins and minerals, the child will unavoidably suffer from a lack of these nutrients and will weigh less than it should considering its genetic makeup.

The weight of the fetus can also be impacted by smoking during pregnancy. Seventy percent of smokers who become pregnant give birth to children who are underweight. The fetus is similarly affected by alcohol, drugs, and even prescription medications that the mother took while she was pregnant. The baby’s body weight is negatively impacted by nervous tension and anxiety, strenuous physical labor, and hazardous work environments.

The expectant mother’s chronic illnesses can also have an impact on the weight of the unborn child. For instance, if you have diabetes, or gestational diabetes, which initially appeared during pregnancy, you run the risk of giving birth to a big, possibly enormous child who weighs more than five kilograms. Additionally, a woman’s chronic kidney, liver, and heart diseases may show symptoms of inadequate fetal weight.

The weight of the child is unavoidably impacted by aspects of the pregnancy. There is nothing to be concerned about if everything goes according to plan. However, a child born under nearly harsh circumstances—against the backdrop of the possibility of miscarriage and other complications associated with childbearing—is not likely to grow up to be a strong man.

Each baby will naturally weigh less than average if a woman is carrying two or three babies during a multiple pregnancy.

How it is determined?

Obstetricians and gynecologists estimate the fetus’s weight using a number of formulas. The mother can figure out how much the baby weighs on her own if you’d like to try.

Lankowitz formula

Although there are mistakes in this calculation method as well, experts believe it to be the most accurate. A woman will need to look in her exchange card on the page of her most recent doctor’s appointment in order to calculate. There she will find her height of the uterine fundus (HFU) and her abdominal circumference (AC), two crucial parameters for the computation.

Both of these values ​​are added together, after which the mother"s weight in kilograms and her height in centimeters are added to the resulting sum. The resulting number is multiplied by 10. For example: the pregnancy period is 30 weeks, abdominal circumference is 110, the height of the fundus of the uterus is 30. The height of the pregnant woman is 170 cm, weight is 70 kilograms. (30 + 110 + 170 + 70) x 10 = 3800. The error in this calculation can be up to 0.5 kg. Therefore, it is reasonable to assume that the woman has a large baby – his real weight at this stage already exceeds 3 kilograms.

Bublichenko formula

All you need to know for the calculation is the expectant mother’s current body weight. After dividing her weight by 200, the result is multiplied by ten. For instance, the weight of a woman is 80 kg. (80/200) x 10 = 4. Because of this method’s significant error, which varies within a kilogram, obstetricians have recently attempted to avoid using it.

Yakubova"s formula

A woman who has made the decision to satisfy her maternal curiosity and learn math skills will require information on the circumference of her abdomen and the height of her uterus’ fundus for these computations. These are located in the exchange card; each scheduled appointment’s data is entered into it. The circumference of the abdomen is multiplied by 100 and divided by 4 after the addition of VDM. VDM-30, for instance, has an abdominal circumference of 100. (30 + 100) / 4 equals 32.5. The result of multiplying by 100 is 3250 grams.

Zhordania"s formula

Once more, the height of the uterus’ fundus and the expectant mother’s abdominal circumference are required for the computation. The sum of these values is multiplied. For instance, when VDM – 30 and circumference – 100 are subtracted, the child’s estimated weight is 3 kg.

Ultrasound

The calculations are thought to be the most accurate sonographic technique, or ultrasound. The Hadlock method is most frequently used to calculate the estimated body weight of the child using special equations that are generated by the scanning apparatus’s software.

The sizes that the instrumental doctor finds on the ultrasound are used as the required mathematical values for the equations. This includes the baby’s head’s biparietal measurement, femur length, and head and abdomen circumferences.

For the purpose of clarity, we will provide the following equations for the two most widely used calculation techniques (as reported by Hadlock and Shepard):

• Shepard – Log10 BW = -1.7492 + 0.166 (biparietal size) + 0.046 (fetal abdominal circumference) – (2.646 [ (abdominal circumference) X (biparietal size) ] / 100).
• According to Hadlock – Log10 BW = 1.3598 + 0.051 (Child"s abdominal circumference) + 0.1844 (length of the femur) – 0.0037 (abdominal circumference X length of the femur), or Log10 BW = 1.4787 + 0.001837 (biparietal head size) 2 + 0.0458 (abdominal circumference) + 0.158 (length of the femur) – 0.003343 (abdominal circumference multiplied by the length of the femur).

Thus, the baby’s measured parameters are combined to form a mathematical expression, from which the computer generates a result. Usually, it is mentioned in the "Estimated fetal weight" section of the ultrasound diagnostic protocol. When carrying twins, the error rises to 15%. The error in a singleton pregnancy is between 8% and 10%.

Although there is currently no more accurate method in medicine, this one cannot be compared to that of a pharmacy.

All current fetal weight norms by weeks and months of pregnancy have been developed using ultrasound data.

Norms at different times

We are all different – tall, short, thin, and plump – so it is hard to say with certainty what the ideal body weight of an unborn child should be. Unborn children have unique qualities as well. As a result, average values persist even though there may be a spread.

The first weight indicators can be calculated by ultrasound machines as early as the eleventh week of pregnancy:

Week-long obstetric phase

Weight estimate, in grams

Week-long obstetric phase

Weight estimate, in grams

The child’s sex was not taken into consideration when compiling this table. Physicians who seek maximum precision when determining delivery technique and other critical matters based on anticipated weight utilize German physicians’ tables, which were created in the early 1990s, to determine the weight of children with European features. It is put together with the fetus’s gender in mind.

Table showing the expected weight in grams for babies of various sexes:

Weeks during the obstetric phase

Weight of a male fetus expected, with allowable variations (grammes)

Weight of a female fetus expected, with allowable variations (grammes)

1690 (1070–2110)

1590 (1010–2050)

1890 (1200–2360)

2130 (1360–2690)

2390 (1600–3000)

2640 (1870–3320)

2550 (1790–3230)

2760 (2060 – 2460)

3090 (2400–3770)

2970 (2290–3650)

3300 (2620–4000)

3160 (2500 – 3850)

3470 (2760–4180)

3320 (2670–4020)

3450 (2800–4180)

There’s a reason why German tables, which are utilized globally, begin at 23 weeks of pregnancy. Boys and girls start to gain weight differently at this point because it is when children of different sexes start to shape their bodies in accordance with gender traits. This difference between babies of different sexes is either absent or not expressed clearly enough in the early stages to be detected for a diagnosis.

Deviations from normal values

A child weighing between 3000 and 3800 g at 39–40 weeks gestation is regarded as normal. On the other hand, neonatologists are unconcerned about newborns weighing more than 4.5 kg or up to 3 kg.

Obstetricians advise expecting mothers not to panic if their actual numbers differ from those shown in the reference tables.

First off, babies grow in "spurts," so at 34 weeks, the baby may upset the mother by falling behind the average norms; by 36 weeks, the baby may even approach the upper limit of the norm. Secondly, you should always account for error—plus or minus 500 g, at the very least.

The fetus is considered large if it regularly outperforms the average during all ultrasound exams performed during pregnancy. In the second trimester, there is already a tendency towards a large baby. It is not possible to draw such a conclusion from a single ultrasound; multiple tests spread over several weeks are needed.

Simultaneously, instrumental measurements are also taken, such as the pregnant woman’s belly circumference and the uterus’ height. Large fetuses typically have VDMs that are higher than the norms (for instance, at 32 weeks, this parameter is increased to 34 cm from the norm of 32 cm, and at 36 weeks, it is approximately 39 centimeters instead of the required 36 cm).

Doctors recommend an additional examination if the fetus shows a noticeable deviation from average values and only "passes" the lowest limit of the norm (5th percentile) during multiple diagnostic measurements. This examination aims to determine the viability of uteroplacental blood flow, potential fetal pathologies (such as chromosomal and hereditary), intrauterine growth retardation, intrauterine infections, and Rh conflict.

Expert-class ultrasound, ultrasonic dopplerography, infection tests, and clinical blood and urine testing are carried out for this reason. If invasive diagnostics and a geneticist consultation are required to find chromosomal abnormalities, the woman is sent.

The decision to consent to such measures rests solely with the expectant mother, particularly in light of the fact that contemporary medicine is prepared to offer a safe but pricy substitute for intrusive diagnostic techniques: a prenatal DNA test derived from the mother’s vein.

When a woman has suspected fetal malnutrition, she is advised to undergo routine cardiotography (CTG) monitoring of the unborn child’s condition. If necessary, she may also need additional treatment, which may involve hospitalization, such as vitamin supplements and medications to enhance uteroplacental blood flow. The baby’s estimated weight is tracked with intermediate ultrasound exams.

If the baby’s parameters are all reduced symmetrically, a deviation from the norm is not deemed especially harmful. Then it might be a typical variation, a characteristic of this child’s anatomy.

Beginning in the seventh month of pregnancy, expectant mothers are highly curious about their unborn child’s weight, and obstetricians need to know this information as well. They make judgments about the ideal way to give birth based on general information about pregnancy and the approximate weight of the child.

A cesarean section is often advised when the fetus is large; even when the baby’s weight is normal, a cesarean section is advised if the fetus’s size does not match the pelvis’ size.

Reliability of determination and errors

As was already mentioned, using ultrasound to estimate the baby’s weight while still inside the mother’s womb is thought to be the most accurate method. Data from Florida-based American physicians indicated that the calculation error made in accordance with Hadlock was roughly 14% of the baby’s body weight. Put another way, there was a 14% discrepancy between the device’s predicted weight and the actual weight. It is the best result to date.

Nowadays, obstetricians hardly use the methods they used before the widespread use of ultrasound, formulas, and measurements because they are no longer necessary. You don’t need to perform any calculations to find out how much the crumbs’ body might weigh. The computer program will assist with all of the calculations. The above formulas’ accuracy, along with a few others’, is estimated to be between 40 and 55 percent. The accuracy of determining the child’s body weight increases by approximately 82% while using ultrasonography.

Weighing is the most accurate method, as gynecologists like to say. Consequently, all you have to do is wait for the birth to find out how much a child weighs in grams. They will provide a thorough and accurate response to this query. Every other approach is similar to telling fortunes. Even with ultrasound technology, a baby’s head and bone size can only be used as a rough indicator of weight because some babies have long legs, like their mother, but are thin or have large heads, like their father.

Doctors themselves claim that the accuracy of determining the weight of the fetus can be affected by the quality of visualization during ultrasound. If a woman is overweight, has extra pounds, fat deposits on the abdomen, then the error will increase. If a woman"s pregnancy occurs against the background of oligohydramnios, the probability of a more significant error than 14% of body weight also increases. It is more difficult to calculate the weight of babies during a twin pregnancy, especially at later stages, because the limbs of one baby on ultrasound can easily be mistaken for the limbs of another, as a result of which a significant error can creep into the forecasts of the body weight of the babies.

It is challenging to estimate a child’s weight if they have developmental pathologies (hydrocephalus, microcephaly, gastroschisis), as the dimensions required to create mathematical equations won’t accurately represent the child’s actual weight.

Being aware of fetal growth is essential to keeping an eye on a healthy pregnancy. You can make sure your baby is growing as expected by monitoring their weight over the course of the weeks. Every developmental milestone, from the earliest stages when the fetus is only a few millimeters long to the later stages when it weighs more than a pound, indicates progress.

Recall that each pregnancy is different. Although there are broad recommendations for fetal weight, individual differences are common. Your healthcare provider’s routine check-ups will provide you with the most accurate information about your baby’s development and general health.

In the end, knowing about these developmental phases makes you feel more invested in your child’s path. Accept the process and have faith in the treatment your medical team is providing. They are there to support you during this amazing time, as well as your infant.

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