Gestational Age, Fetal Development & Due Dates: Everything You Need to Know
What Is Gestational Age?
Gestational age (GA) is a common term used to describe the age of a pregnancy. It is measured in weeks, usually by calculating the time difference between the first day of the woman’s last menstrual cycle and the current date.
Here’s the part that surprises many people: you’re actually not pregnant the first week or two of what’s counted as your pregnancy’s 40 weeks. Conception typically happens about two weeks after the last period begins. But to find your estimated due date, your healthcare professional counts ahead 40 weeks from the start of your last period — meaning the period itself is counted as part of your pregnancy timeline, even though you weren’t pregnant at the time.
Gestational age is counted from the LMP (last menstrual period). Fetal or conceptional age is counted from conception and is usually about 2 weeks less than gestational age.
How Is Your Due Date Calculated?
The due date is usually estimated by calculating the date that is 40 weeks from the start of a woman’s LMP, under the assumption that childbirth on average occurs at a gestational age of 280 days.
There are several ways your provider may date your pregnancy:
Last Menstrual Period (LMP): The classic method, though the LMP method assumes a 28-day cycle, which may not apply to women with shorter or longer cycles. Women with irregular cycles or those who don’t ovulate on day 14 may experience discrepancies between their calculated and actual due date.
Ultrasound: First-trimester ultrasound is the most accurate ultrasound method for establishing or confirming gestational age. Measurement of the embryo or fetus early in pregnancy can estimate gestational age within several days. Later ultrasounds remain important for assessing growth and anatomy but are less precise for redating a pregnancy.
IVF: When using in vitro fertilization, the estimation of the due date is generally more precise than calculating the due date based on natural conception, since the exact transfer date is known.
Can due dates change? Yes. Following your ultrasound scan, your EDD may be revised if the ultrasound-based EDD differs from the initial EDD by a designated amount for the gestational age of the fetus at the time of the scan. In the earlier stages of pregnancy, fetal growth is relatively uniform. As a practical note, only about 5% of babies are actually born on their exact due date, with most coming within a week or two before or after the EDD.
Understanding Pregnancy Terms
Pregnancy is measured in gestational weeks starting from the first day of your last period. Delivery timing is classified according to ACOG as:
Early term: 37 0/7 through 38 6/7 weeks
Full term: 39 0/7 through 40 6/7 weeks
Late term: 41 0/7 through 41 6/7 weeks
Postterm: 42 0/7 weeks and beyond
Fetal Development: Trimester by Trimester
Each trimester marks a distinct phase of fetal development: the first trimester (weeks 1–12) involves the initial formation of essential organs and systems; the second trimester (weeks 13–26) brings rapid growth and sensory development; and the third trimester focuses on preparing the baby for life outside the womb.
🌱 First Trimester (Weeks 1–12)
The first trimester is generally when most people develop unpleasant symptoms like morning sickness and fatigue. During this trimester, the fertilized egg will change from a small grouping of cells to a fetus that begins to have human features.
Key milestones include:
The neural tube forms, which becomes your baby’s brain, spinal cord and backbone. Tiny buds start to appear that will become arms and legs. Early cardiac activity begins during the first several weeks of embryonic development and can often be detected on ultrasound during early pregnancy.
By week 6, the neural tube along your baby’s back is closing. The brain and spinal cord develop from the neural tube. The heart and other organs also start to form.
By the end of the first trimester, the major organ systems have formed, although they continue to grow and mature throughout pregnancy. The risk of miscarriage drops significantly after this point. The placenta has taken over hormone production from your ovaries. By week 12, your baby’s fingerprints are already forming — a unique pattern that will remain unchanged throughout life!
🌿 Second Trimester (Weeks 13–26)
The second trimester is often thought of as the best part of the experience. By this time, any morning sickness is probably gone. You may also start to feel movement as the fetus flips and turns in your uterus. Many people find out about the fetus’s assigned sex during an anatomy scan at around 20 weeks. The fetus can even suck its thumb, yawn, stretch, and make faces.
Key milestones include:
By week 16, the baby’s limb movements are becoming more coordinated and can be seen during ultrasound exams. By now, your baby might be about 4¾ inches long from crown to rump and weigh about 4 ounces.
By week 18, your baby’s ears start to stand out on the head, and the baby might begin to hear sounds. The digestive system has started working.
During the periviable period—roughly the early-to-mid 20s weeks of gestation—survival outside the womb becomes possible with intensive neonatal care, but outcomes vary greatly with each additional week of gestation and with the baby's individual circumstances.
By week 26, your fetus can open their eyes. The retinas are developing, and the baby may respond to bright lights. Brain development continues rapidly, with brain waves becoming more complex.
‘🍂 Third Trimester (Weeks 27–40)
Your baby’s nervous system is developing quickly. Your baby adds more fat to her body, which makes her skin look smooth and less wrinkly. Your baby’s body is making melanin, a substance that gives skin color and protects it from the sun after birth.
Key milestones include:
By around 32 weeks, many fetuses weigh roughly 3–4 pounds, although normal size varies considerably.
By weeks 35–36, Substantial brain growth continues during the final weeks of pregnancy; a baby's brain at 35 weeks is still considerably less mature than it will be at full term. Most babies move into a head-down position to get ready for labor and birth.
Brain Development: The Long Game
The development of the fetal brain is a complex and ongoing process that occurs throughout pregnancy. Brain development occurs throughout pregnancy. The third trimester is a particularly rapid period of brain growth and organization, while important structural development begins much earlier. During this time, the brain undergoes significant growth, structural organization, and maturation — including neural proliferation, neuronal migration, synaptogenesis, and myelination.
🗺️ Gestational Age Maps, Charts & Interactive Tools
Here are the best online resources for tracking gestational age and fetal development visually:
Interactive Fetal Development Maps:
MotherToBaby Critical Periods Tool — Enter the first day of your last menstrual period and this interactive tool shows when different parts of your baby’s body form during pregnancy, and when birth defects or pregnancy complications might happen — including when the baby’s body parts could be most sensitive to exposures.
Perinatology Fetal Development Chart — Includes a gestational age calculator and critical periods chart for each organ system.
Clinical Fetal Growth Charts:
INTERGROWTH-21st Charts & Calculators — Free downloadable charts for gestational age estimation (8–14 weeks), fetal growth (14–40 weeks), and umbilical artery Doppler indices (24–40 weeks), used globally by healthcare professionals.
WHO Fetal Growth Calculator — WHO-based standards for estimating fetal growth percentiles by sex and gestational age.
Pregnancy is a remarkable 40-week journey, and knowing how gestational age works helps you stay connected to every milestone. Always work with your healthcare provider to confirm dating and interpret any growth measurements — these tools are powerful guides, but your clinician’s assessment remains the gold standard.
Sources & Clinical References
American College of Obstetricians and Gynecologists. Methods for Estimating the Due Date. Committee Opinion No. 700. Obstetrics & Gynecology. 2017.
American College of Obstetricians and Gynecologists. Definition of Term Pregnancy. Committee Opinion.
American College of Obstetricians and Gynecologists; Society for Maternal-Fetal Medicine. Periviable Birth.Obstetric Care Consensus.
Centers for Disease Control and Prevention. About Birth Defects and Neural Tube Defects.
MotherToBaby. Critical Periods of Development.
INTERGROWTH-21st fetal growth standards, if you keep fetal-size references.
This is something that often causes confusion, even for parents who have done a lot of research before arriving in the NICU.
Dates and developmental timelines don’t always feel straightforward in real-life situations.
The most helpful approach is to focus on what your baby needs right now rather than trying to map everything perfectly to a timeline.
If you’re looking for a clearer starting point, the First 7 Days Survival Guide can help you understand what matters most early on.
Jeri Power, MSN, APRN, NNP-BC
