Fetal Growth Rates: What to Expect Week by Week

Fetal growth rate

Hello, Dr.
Lu.
Could you please introduce the growth rate of the fetus, such as the approximate size in centimeters during specific weeks? I have found significant discrepancies in various books I consulted, so I would like to ask for the standard assessment criteria used by general practitioners for reference.
Additionally, my doctor estimated 5 weeks on October 1 and 8 weeks on November 1.
Why is there such a discrepancy? If the ultrasound shows approximately 20.5 mm, how large is the fetus at that stage? Thank you for answering these questions.

gogo, 20~29 year old female. Ask Date: 2002/11/11

Dr. Lü Lizheng reply Obstetrics and Gynecology

Hello! 1.
In a standard ultrasound, the distance from the fetal head to the buttocks is used as a measurement standard.
Typically, the fetal legs are bent towards the abdomen, so only the distance to the buttocks is measured.
Generally, the sizes are as follows: at 6 weeks, it measures 0.3 cm; at 7 weeks, 0.9 cm; at 8 weeks, 1.5 cm; at 9 weeks, 2.2 cm; and at 10 weeks, 3.0 cm.
Therefore, 20.5 mm corresponds to the size of a fetus at 8 weeks and 5 days.
2.
The measurement can be affected by whether the fetal back is straightened during each measurement.
Additionally, from 6 weeks onward, the gestational age is calculated based on the size of the gestational sac, which can also lead to discrepancies.
Wishing you and your family peace and happiness.
Sincerely, Dr.
Lu Li-Cheng, Department of Obstetrics and Gynecology, Taoyuan Hospital, Ministry of Health and Welfare.

Reply Date: 2002/11/11

More Info

Understanding fetal growth rates is crucial for expectant parents to monitor the development of their baby throughout pregnancy. Fetal growth is typically assessed in terms of gestational weeks, starting from the first day of the last menstrual period (LMP). This method is standard practice in obstetrics, although some practitioners may also consider the date of conception or ovulation, especially when there are irregularities in menstrual cycles.

In general, fetal growth can be summarized as follows:
1. Weeks 1-4: During this period, the embryo is developing rapidly, but it is still very small, typically measuring only a few millimeters. By the end of week 4, the embryo is about 1-2 mm in size.

2. Weeks 5-8: By the end of week 5, the embryo measures about 1 cm. By week 8, it has grown to approximately 1.6 cm. This is a critical time for organ development.

3. Weeks 9-12: The embryo is now referred to as a fetus. By the end of week 12, the fetus is about 6-7 cm long and has developed most of its major organs.

4. Weeks 13-16: The fetus continues to grow rapidly, reaching about 10-12 cm by week 16. This is when many parents may start to see the first ultrasound images.

5. Weeks 17-20: At this stage, the fetus measures about 15-20 cm. The development of facial features becomes more pronounced, and movements may be felt by the mother.

6. Weeks 21-24: The fetus grows to about 26-30 cm by week 24. This is a period of significant growth and development, including the maturation of the lungs and brain.

7. Weeks 25-28: By week 28, the fetus can measure around 35-38 cm. The body fat begins to accumulate, and the fetus becomes more active.

8. Weeks 29-32: The fetus continues to grow, reaching about 40-45 cm by week 32. This is when the fetus starts to position itself for birth.

9. Weeks 33-36: By week 36, the fetus can measure around 48-50 cm. The body is preparing for delivery, and the fetus may begin to engage in the pelvis.

10. Weeks 37-40: At full term (week 40), the average fetus measures about 50-53 cm and weighs around 3-4 kg.
Regarding the discrepancies in gestational age calculations, it is important to note that different methods of estimating gestational age can lead to variations. For instance, if your doctor calculated 5 weeks on October 1 and then 8 weeks on November 1, this could be due to differences in how they interpreted your LMP or the timing of ovulation. The ultrasound measurements can also provide a more accurate estimate of gestational age, especially if they are taken during the first trimester when fetal growth is more uniform.

When you mention that an ultrasound showed a measurement of approximately 20.5 mm, this typically corresponds to a gestational age of around 7 weeks. However, the exact correlation can vary based on individual growth patterns. It's essential to have regular follow-ups with your healthcare provider to monitor fetal growth and development accurately.

In conclusion, while there can be variations in fetal growth rates and gestational age calculations, regular ultrasounds and blood tests can provide a clearer picture of your baby's health and development. If you have concerns about discrepancies in measurements or growth rates, discussing them with your healthcare provider can help clarify any uncertainties.

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