A developing baby is officially classified as a fetus starting 9 weeks after the last menstrual period, or about 7 weeks after fertilization. Before that point, it passes through two earlier stages: a single-cell zygote in the first few days, then an embryo for the following several weeks. By the time it reaches the fetal stage, the basic framework of every major organ system is already in place, and the remaining months are devoted to growth and maturation.
Two Ways to Count: Gestational Age vs. Conception Age
One of the most confusing parts of pregnancy timelines is that there are two different clocks. Gestational age counts from the first day of the last menstrual period, which is roughly two weeks before conception actually happens. Conception age (also called embryonic or fetal age) counts from fertilization itself. So when a source says “9 weeks,” it matters which system they’re using. In general, gestational age is about two weeks ahead of conception age. Most doctors and pregnancy apps use gestational age, so that’s what you’ll see on ultrasound reports and due date calculations.
This article uses gestational age unless stated otherwise, since that’s the standard in clinical settings.
Days 1 Through 5: From Fertilization to Blastocyst
The moment a sperm fertilizes an egg, the resulting single cell is called a zygote. Within 24 hours it begins dividing. By day 2 or 3, it has become a compact ball of cells called a morula, which then hollows out into a blastocyst. Around day 4, the blastocyst sheds its outer shell. By day 5 it is free-floating in the uterus, preparing to implant in the uterine wall. This entire sequence unfolds while the cluster of cells is still traveling down the fallopian tube.
Weeks 1 Through 8: The Embryonic Period
Once implantation occurs (around gestational week 4), the developing organism is called an embryo. The next several weeks are the most transformative stretch of the entire pregnancy. A process called organogenesis runs from roughly weeks 3 through 8, during which every major organ system takes shape: the heart, brain, lungs, liver, kidneys, and digestive tract all begin forming from scratch.
The neural tube, which eventually becomes the brain and spinal cord, folds and closes during weeks 3 and 4. This is why folic acid intake before and during early pregnancy is so important. If the tube doesn’t close properly, it can lead to conditions like spina bifida. The earliest involuntary movements begin around week 7, though they’re far too small for anyone to feel.
A heartbeat can be detected as early as 6 weeks with a transvaginal ultrasound, though detection rates are more reliable a few weeks later. At 8 to 9 weeks, a transvaginal Doppler picks up cardiac activity in about 60% of pregnancies. By 9 to 10 weeks, that rate climbs to nearly 88%. A standard abdominal ultrasound typically detects cardiac activity starting around week 7, with reliability increasing after that.
By the end of week 8, the embryo’s organ systems have developed their initial structures and are ready for further maturation. Fingers and toes are forming and beginning to separate. The embryo at this point is still tiny, but it has a recognizably human shape.
Week 9: The Transition to Fetus
At 9 weeks gestational age (7 weeks after fertilization), the embryo is reclassified as a fetus. This isn’t an arbitrary label change. It marks the point where the basic blueprint of the body is complete and development shifts from building new structures to refining and growing existing ones. At this stage, the fetus measures about 22 millimeters from crown to rump, roughly the size of a cherry.
From here, organs that were laid down during the embryonic period begin maturing. The liver starts producing blood cells. Bones begin hardening from cartilage. The kidneys start functioning. The brain, which will continue developing well after birth, begins a long period of rapid growth and specialization.
Weeks 10 Through 27: The Second Trimester and Beyond
During the second trimester, the fetus grows from a few centimeters to roughly 14 inches long. This is when you can typically start to feel movement, usually between weeks 16 and 25 for a first pregnancy, sometimes earlier for subsequent ones. The fetus develops fingerprints, begins practicing breathing movements with amniotic fluid, and its hearing matures enough to respond to sound.
Sex organs become distinguishable on ultrasound around weeks 18 to 20. Fat stores begin building under the skin, and the lungs produce a substance that will eventually help them inflate after birth. The eyes, which formed during the embryonic period, develop the ability to sense light.
Weeks 28 Through 40: Final Growth and Preparation
The third trimester is primarily about weight gain and organ maturation. The fetus roughly doubles or triples its weight during this period, going from about 2 pounds at week 28 to an average of 6 to 9 pounds at full term. The brain undergoes its most dramatic growth phase, forming the deep folds and connections that support complex function after birth.
Lung maturity is one of the last major milestones. The lungs aren’t considered fully mature until around 36 weeks, which is a key reason premature births before that point carry higher risks. The fetus also shifts position during the final weeks, ideally moving head-down in preparation for delivery.
How Long the Full Process Takes
From fertilization to fetus takes about 7 weeks (9 weeks gestational age). From fertilization to a full-term birth takes approximately 38 weeks, or 40 weeks by gestational age. The most critical window is the first 8 weeks after fertilization, when all major organs are forming and the developing organism is most vulnerable to disruptions. After that transition to the fetal stage, the remaining 30 or so weeks are focused on growth, maturation, and preparing the body for life outside the womb.