A correct description of fetal development states that the process occurs in three distinct stages (germinal, embryonic, and fetal), with the embryo officially becoming a fetus at the ninth week of pregnancy and continuing to grow and mature until birth. If you encountered this question on a test or quiz, that core fact is what most correct answer choices hinge on. Below is a detailed walkthrough of what actually happens during each stage so you can evaluate any statement about fetal development with confidence.
The Three Stages of Prenatal Development
Prenatal development is divided into three stages: germinal, embryonic, and fetal. The germinal stage covers roughly the first two weeks after fertilization, when the fertilized egg divides rapidly and implants in the uterine wall. The embryonic stage runs from about week three through the end of week eight. During this period, all major organ systems begin to form, which is why it’s considered the most critical window for structural development. At the ninth week of pregnancy, the embryo is reclassified as a fetus. The fetal stage then lasts from week nine until birth.
This transition point is one of the most commonly tested facts in biology and health courses. After the eighth week, healthcare providers refer to the developing organism as a fetus. It keeps that designation for the remainder of the pregnancy.
What Happens During the Embryonic Stage
The embryonic stage is when the basic blueprint of the body takes shape. Cardiac tissue begins to pulse at around five to six weeks of pregnancy, registering on ultrasound as a heartbeat even though the heart itself is still forming. By week 10, the fetal heart is fully developed with four distinct chambers. The neural tube, which becomes the brain and spinal cord, closes during the first month. By about six weeks (42 days from conception), brain wave activity can be detected, and the jaw, early teeth, and taste buds have started to form.
This is why the embryonic period is so sensitive to disruption. Nearly every organ system is being established during these few weeks. A substance or infection that interferes with development here can have outsized effects compared to the same exposure later in pregnancy.
Growth and Refinement in the Fetal Stage
Once the fetal stage begins at week nine, the focus shifts from forming new structures to growing and refining the ones already in place. Organs become more complex, bones harden, and the fetus gains weight steadily.
Around 15 weeks, a massive wave of nerve cell production begins in the brain and continues for about a month. By five months, each side of the brain contains roughly a billion nerve cells. Eye movements appear on ultrasound as early as 14 to 15 weeks. By 26 weeks, the eyelids are partially open. By 28 weeks, the eyes are fully open, and by 31 weeks the pupils can constrict, dilate, and detect light. From that point on, the fetus has the capacity for directed vision if enough light reaches it.
Mothers typically feel the first fetal movements, called quickening, between 16 and 20 weeks. Women who have been pregnant before often notice it closer to 16 weeks, while first-time mothers commonly don’t feel movement until around 20 weeks. Most describe the sensation as bubbles popping or light tapping.
Lung Development and the Question of Viability
The lungs are one of the last organs to mature, which is a major reason extremely premature births are so dangerous. Storage structures for surfactant, the slippery substance that keeps air sacs from collapsing, begin appearing around week 20. Surfactant production starts at about 24 weeks, but the lungs don’t produce enough of it to function reliably until around 32 weeks. This timeline is central to understanding fetal viability.
Medical viability, the point at which a fetus could potentially survive outside the womb with intensive care, is generally placed around 22 to 24 weeks. Survival rates at 22 weeks with active medical intervention range from about 30 to 35 percent in recent U.S. data, though some specialized centers report higher figures. By 24 to 26 weeks, those odds improve significantly. A few centers have even begun offering resuscitation at 21 weeks in select cases, with one reporting 35 percent survival to discharge among a small group of 17 infants. These are still exceptional outcomes, not the norm, and survival at these early ages often comes with serious health challenges.
Common Statements and Why They’re Right or Wrong
If you’re evaluating a multiple-choice question, here are the key facts that separate correct statements from incorrect ones:
- The embryo becomes a fetus at week nine. This is correct. Any statement placing that transition earlier or later is wrong.
- The fetal stage is the longest stage. Correct. It runs from week nine until birth, spanning roughly 30 weeks compared to about two weeks for the germinal stage and six weeks for the embryonic stage.
- Major organs form during the embryonic stage. Correct. The embryonic period is when the foundational structures of the heart, brain, lungs, and other organs are established.
- The fetal stage is primarily about growth and maturation. Correct. Organs that were laid down during the embryonic stage continue to develop and become functional.
- The heart begins beating during the fetal stage. Incorrect. Cardiac activity starts at five to six weeks, which falls within the embryonic stage.
- Lungs are fully mature at birth. Technically incomplete. Lung development continues after birth, with the number of air sacs increasing until about age three and full lung maturation not occurring until around age eight.
The most reliably correct statement in any standard biology or health course will emphasize that fetal development moves through three sequential stages, that organ formation happens primarily during the embryonic period, and that the fetal stage is defined by continued growth and maturation of structures already in place.