Three months pregnant signals the completion of the first trimester, typically spanning weeks nine through twelve. This period is characterized by rapid development as the developing life transitions from foundational structures to a recognizable human form. While the physical size remains small, internal changes establish the framework for all future growth. This stage represents a milestone, moving toward a period of greater stability.
Baby Size and Measurement at 3 Months
The baby’s size is measured using the Crown-Rump Length (CRL). At the beginning of the third month, around Week 9, the length is approximately 22 millimeters, similar in size to a grape. Weight is negligible, often less than two grams.
By Week 10, the CRL has increased to about 3.2 centimeters, roughly the size of a small green olive or a kumquat. The head remains disproportionately large, making up almost half of the total length, as the brain continues its rapid development.
By the end of Week 12, the baby measures approximately 61 millimeters in length, comparable to a lime or a small plum, and weighs between 14 and 20 grams. This growth rate demonstrates the internal activity taking place before the second trimester begins.
Key Developmental Milestones
The third month is marked by the refinement of structures formed during the earlier weeks of the first trimester. By Week 10, the webbed fingers and toes have separated, and the limbs can bend at the elbows and knees. The beginnings of fingernails, toenails, and the external ears are taking on their final shape.
Internally, major organ systems begin to function. The circulatory and urinary systems are working, with the kidneys starting to filter toxins and the liver producing bile. The digestive tract is developing, with the intestines starting to rotate into their proper position within the body cavity.
Reflex movements are initiated during this period, including the ability to open and close the fists, curl the toes, and make sucking motions. While these movements are not yet strong enough for the parent to feel, they can be observed during an ultrasound scan. The skeleton, which was primarily cartilage, begins the process of ossification, with hard bone tissue replacing the softer material.
The Shift from Embryo to Fetus
The transition that occurs around Week 10 shifts the classification from an embryo to a fetus. The embryonic stage is defined by the formation of all the major organ systems and body structures. By the time the fetal stage begins, over 4,000 distinct anatomical structures have already appeared.
The fetal stage, which extends until birth, is dedicated to growth and the maturation of these established organs and systems. This change in terminology signifies that the most structurally vulnerable period of development is largely complete. After Week 12, the chance of miscarriage drops considerably. The baby’s sex is determined genetically from conception, but the external genitals begin to form during this period, though they are usually not distinguishable on an ultrasound until later.
Maternal Changes During the Third Month
As the first trimester concludes, many parents experience a shift in the intensity of early pregnancy symptoms. Morning sickness and nausea begin to subside during the latter half of the third month. While fatigue may persist, hormonal stabilization that accompanies the end of the first trimester brings a slight increase in energy.
The uterus, which was previously the size of a fist, has grown to roughly the size of a grapefruit by Week 10. Although a baby bump may not be prominent for everyone yet, the parent’s clothing may start to feel snug around the waist. Increasing blood volume can lead to more visible veins and an increase in clear vaginal discharge. This period is when a healthcare provider can first detect the baby’s heartbeat using a Doppler device during a routine appointment.