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The foetus grows right from the earliest days of pregnancy. Ultrasound scans are done at regular intervals during pregnancy and help determine the approximate weight and length of the baby. The growth of the foetus in the womb leads to the increase in the size of the bump too.
Foetal Weight Chart
The table below shows the average increase in the weight of the foetus on a week by week basis. The numbers are only averages, and the actual weight and length of the foetus may vary significantly. These are general guidelines and your baby’s weight may not be an exact match to what you see in the table. However, this should not be a cause for worry.
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Foetal Length Chart
Along with the weight, the doctor keeps a record of the foetal length as well. This chart is a rough guide of the foetal length on a week by week basis. The length of the foetus is measured from the crown till the bottom, because of the position he’ll be in, i.e., legs curled up against the torso, which makes it difficult to measure his length from head to toe. From the 20th week onwards, the length of the foetus will be calculated from the crown to the heel.
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Ultrasonic Foetal Measurement Standards
The gestational age chart is used to determine the biometric measurements and determine if the foetal growth is normal. If the mother-to-be is able to provide the date of the last menses before conception, it gives a much better estimate of EDD, which is the estimated date of confinement (also known as Expected Date of Delivery). The EDD can be estimated to plus or minus 5 days.
The method of using ultrasound scans to estimate the gestational age is done under the assumption that the baby is growing normally and as per the foetus weight chart. If any condition alters the growth of the foetus, it will also make the estimate less reliable. The ultrasound method lets one measure the number of foetal and embryonic structures, but only a few of them are reliable and easy to carry out repeatedly. The ultrasound foetal measurements are done using various foetal graphs and calculators.
Estimated Foetal Weight Calculator
The foetal weight calculator lets you calculate the percentile of the foetal weight as well as the estimated foetal weight at the time of delivery. The inputs that you need to enter are the gestational age, biparietal diameter, humerus, head circumference, femur length, and the abdominal circumference. The calculator then calculates the percentile for each factor, as well as the estimated foetal weight at the time of delivery.
Suppose the foetus is in the 60th percentile for weight, at 30 weeks. This means that 60% of foetuses weigh the same or less than the baby at 30 weeks. This means that 40% of foetuses weigh more than the baby at 30 weeks.
It is a common practice to opt for an ultrasound scan to estimate the weight of the foetus towards the end of pregnancy. The accuracy rate of this method is very low and the calculated weight may lead to changes in the way the mother and the physician may perceive the delivery. Therefore, the method has been questioned in medical circles, and is not recommended until there is a medical condition.
The foetal weight calculator calculates the foetal weight as well as the estimated foetal weight at the time of delivery as per ultrasound data using many formulas like Warsof, Shepard, Hadlock, Campbell, and so on.
The calculation is carried out using the four basic parameters which are: head circumference (HC), biparietal diameter (BPD), abdominal circumference (AC) and femur length (FL).
The calculated foetal weights may be 16% plus or minus the average weight. The actual weight can be either higher or lower than 16%.
Around 68% of babies will be within the range of 1 standard deviation (SD), and 2 SD should encompass around 95% of the actual weight. However, about 5% of the babies will be either higher or lower than 2.5%, thus making this an inaccurate calculator of foetal weight.
Foetal Growth Percentile Calculator
The growth percentile calculator calculates how big or small the baby is as compared to the average value. If the baby is in the 40th percentile for growth, this means that the baby is larger than 40% of babies who are of the same age, and smaller than 60% of babies of the same age. 50 is the average value and a value below 50 means that the baby’s growth is below the average value. A value above 50 means that the baby has an above-average growth rate.
A foetus that measures between the 10th and the 90th percentile is considered normal.
The different terms used in tracking or calculating the growth of the foetus in the womb are briefed upon below for better understanding:
The embryonic crown-rump length is a parameter that dates the embryo and estimates the foetus’ average length. It can be measured to an accuracy of plus or minus 3 to 5 days. The limbs and the yolk sac are excluded from this calculation. The crown-rump length can also be used to calculate the gestational age, wherein doctors can estimate your due date.
Biparietal Diameter and Head Circumference
The BPD or the Biparietal Diameter is the transverse width of the head at the widest part. The measurement is done from one leading edge to the other leading edge of the bone on the opposite side, as it is the most distinct. The scope for error in this measurement is not a lot as the oval shape of the head does not leave a lot of room for mistakes. This makes it easy to use the method over and over again. The size of the head is determined by the growth of the brain and does not take into account other aspects of the brain’s development.
Femur and Humerus Length
The femur length method is also reliable and can be repeated. Skeletal dysplasias tends to affect it but this condition is very rare. The measurement confirms the head’s measurement and the best value can be obtained after 14 weeks. To estimate the gestational age, at least 2 or sometimes even 4-5 measurements may be required.
Estimated Foetal Weight
There are different types of computer software and charts that help the doctor calculate the foetal weight, along with measurements such as the abdominal circumference, biparietal diameter, the femur length and so on.
This is a measurement taken in the second half of the pregnancy and is used to assess foetal growth. It is an approximate method and it is used to see if the growth proportions of the foetus are normal. It is measured at the level of the liver and the stomach along with the left portal vein at the umbilical region.
This is measured in early pregnancy and is measured in three dimensions. The ‘Mean Sac Diameter’ is the average that is used to estimate the gestational age. The method can be applied between the 5th and the 8th week of pregnancy and has an accuracy of plus or minus 3 days.
The gestational sac surrounds the embryo and can be seen in the early ultrasound scans. The sac encloses the embryo as well as the amniotic fluid. It allows the doctor to measure the number of weeks of pregnancy (with a margin of plus or minus 5 days).
This is a membranous sac and is attached to the embryo. The sac can be spotted between the embryo and the gestational sac. It offers nourishment to the embryo before the circulatory system and the placenta develops to give the foetus the required nourishment.
This is a mass of cells and can be seen before the embryo becomes visible. The rate of growth is approximately 1 mm a day and starts at the 6th week of the gestational age. It is used to date an early pregnancy and also, to estimate the gestational week.
The gestational age is calculated in most cases by using BPD, femur length, head circumference, and abdominal circumference. It may not be very important to see how they are combined. The modern ultrasound machines have a computer biometric analysis program that calculates the estimated due date.
The doctor may conduct a multiparameter assessment of the gestational age after the first trimester of pregnancy. The basic ones are the biparietal diameter and the femur length. The other parameters such as occipitofrontal diameter, head circumference or humerus length are also considered.
The early measurement to calculate gestational age is usually taken as a definite assessment. There is a reduced accuracy after 20 weeks. The doctor would evaluate further if the ultrasound foetal measurement standard parameters are not in agreement at any time. This is done to average all the values to get an estimated gestational age.
Please note that these are the international standards of weight and length using guidelines from the UK. The average weight and length of an Indian baby may differ from the international standards.
The foetal growth chart is an important indicator of the progress of the pregnancy and the growth of the baby. Any anomaly detected during the regular measurements is addressed by the doctor to see if intervention is necessary. It is advisable to go for all the mandatory checkups and scans during pregnancy to ensure that the charts are tracked correctly.