Written by: Dr. Shahrukh Khan Bahadur, MBBS, Fellowship in Neuro
Medically reviewed by: Dr. Pooja Singh, MBBS, MS (Obs & Gynae), Consultant Obstetrician and Gynecologist
Last reviewed: September 24, 2026
Pregnancy is a gradual process, and your baby goes through many changes from the early weeks until birth. Understanding fetal growth by week can help you learn how your baby develops during each stage of pregnancy.
Every pregnancy is different, so the timing and pace of development can vary. A pregnancy calculator can help estimate your pregnancy week and due date, while your healthcare professional can provide personalized information about your pregnancy.
What Is Fetal Growth?
Fetal growth refers to the physical development of your baby during pregnancy. It includes changes in size, weight, organs, bones, muscles, the nervous system, and other parts of the body.
Pregnancy is usually measured from the first day of the last menstrual period. This means pregnancy dating includes approximately two weeks before conception normally occurs.
Fetal Growth During the First Trimester
Weeks 1 to 4
During the first few weeks, ovulation, fertilization, and implantation take place.
After fertilization, the developing embryo travels toward the uterus and implants in the uterine lining. Early structures that support the pregnancy begin developing.
At this stage, the pregnancy is extremely small and may not yet be visible on an ultrasound.
Weeks 5 to 8
Important early development takes place during these weeks.
The neural tube begins developing into the brain and spinal cord. The early heart and other important structures also begin to form. Small limb buds appear and gradually develop into the arms and legs.
Weeks 9 to 12
The developing baby is now generally called a fetus rather than an embryo.
The face becomes more defined, fingers and toes continue developing, and major organs continue to mature. Movement may begin, although you usually cannot feel these movements yet.
By the end of the first trimester, many of the baby’s basic body structures have formed and will continue developing throughout pregnancy.
Fetal Growth During the Second Trimester
Weeks 13 to 16
Growth becomes increasingly noticeable during the second trimester.
Bones continue developing, facial features become more defined, and the fetus begins making more coordinated movements. The nervous system and other organs continue to mature.
Weeks 17 to 20
The fetus continues to grow in length and weight.
Movements become stronger and more coordinated. Some women may begin feeling their baby move during this period, although the timing varies from person to person.
An ultrasound around the middle of pregnancy may be used to examine fetal development and assess the baby’s anatomy.
Weeks 21 to 24
The fetus continues gaining weight while the brain, lungs, and other organs develop further.
Movements may become easier to notice. Hearing also continues to develop as the pregnancy progresses.
Regular prenatal appointments remain important for monitoring the health of both the mother and baby.
Fetal Growth During the Third Trimester
Weeks 25 to 28
The fetus continues growing and gaining weight.
The brain develops further, the lungs continue maturing, and more body fat develops. Fetal movements may become more noticeable.
Weeks 29 to 32
The fetus continues gaining weight and strength.
The brain and nervous system mature further, while the lungs continue developing. As the baby grows and space becomes more limited, the type of movement you notice may change.
Weeks 33 to 36
Growth continues as the baby prepares for birth.
The lungs and other organs continue maturing, and the fetus gains additional body fat. Many babies gradually move into a head-down position, although this does not happen at exactly the same time in every pregnancy.
Weeks 37 to 40
The pregnancy is approaching or reaching full term.
The fetus continues gaining weight and completing the final stages of development. Not every baby is born exactly on the estimated due date.
How Is Fetal Growth Monitored?
Healthcare professionals may monitor fetal growth using several methods, depending on the stage of pregnancy and individual circumstances.
These may include:
- Fundal height measurements
- Ultrasound examinations
- Fetal heart rate monitoring
- Maternal health assessment
- Pregnancy history
- Assessment of fetal movements
- Additional tests when medically appropriate
Ultrasound measurements can help estimate fetal size, but they are not exact measurements of the baby’s actual weight.
Does Every Baby Grow at the Same Rate?
No. Babies do not all grow at exactly the same rate.
Fetal measurements can vary because of genetics, gestational age, maternal health, placental function, and normal individual differences.
A measurement that differs from an average reference does not automatically mean that there is a problem. Healthcare professionals consider the overall growth pattern together with other clinical information.
What Is a Fetal Growth Calculator?
A fetal growth calculator is an online tool that may provide an estimate of fetal size or growth based on pregnancy information and, depending on the tool, ultrasound measurements.
These tools can be useful for general educational information, but they should not replace prenatal appointments or professional medical assessment.
If you want to estimate your pregnancy week and expected due date, you can use our Pregnancy Calculator.
When Should You Contact a Healthcare Professional?
Speak with your healthcare professional if you have concerns about your pregnancy or your baby’s development.
Seek prompt medical advice if you experience symptoms such as:
- Vaginal bleeding
- Severe or persistent abdominal pain
- Severe headache or vision changes
- Fluid leaking from the vagina
- Significant or sudden swelling
- Noticeably reduced or changed fetal movements after you have established a usual movement pattern
If you are unsure whether a pregnancy symptom is normal, contact your healthcare or maternity team for advice.
Fetal Growth and Pregnancy Weeks
Knowing your pregnancy week provides a general framework for understanding fetal development. However, pregnancy dating is an estimate, and your healthcare professional may adjust the estimated due date based on clinical assessment or ultrasound findings.
You can use our Pregnancy Calculator to estimate your pregnancy week and due date.
Frequently Asked Questions
What is fetal growth by week?
Fetal growth by week describes the general changes that occur as your baby develops from the early weeks of pregnancy through birth.
When does fetal development begin?
Development begins after fertilization. Pregnancy dating, however, is usually calculated from the first day of the last menstrual period.
Can fetal growth be measured accurately?
Ultrasound can provide useful measurements of fetal development, but estimates of fetal size and weight are not exact.
When can I feel my baby move?
Many women begin noticing fetal movement during the second trimester. The timing varies between pregnancies.
Does fetal growth continue until birth?
Yes. Your baby continues developing and gaining weight throughout pregnancy, including during the final weeks before birth.
Final Thoughts
Fetal growth changes continuously throughout pregnancy. From early organ development to increasing movement and weight gain near birth, each stage contributes to your baby’s development.
Information from pregnancy calculators and online articles can help you understand pregnancy dates and general fetal development, but they cannot replace prenatal care. If you have concerns about fetal growth or your pregnancy, speak with a qualified healthcare professional.