Pregnancy can be a nerve-wracking experience for everyone involved. So, whether you or someone you know is pregnant, you might wonder what changes are happening inside a woman’s body. To appease your curiosity, let’s look at how each of us grew in the first 12 weeks of conception.
Fertilization: the first two weeks
A normal pregnancy is 40 weeks and begins from the first day of the woman’s last menstrual period (LMP). Conception, also known as fertilization, usually takes place two weeks after a woman’s LMP during ovulation. Following conception, implantation takes place, and the now fertilized egg attaches to the wall of the uterus. This period of development is scientifically known as the germinal stage.
Circulatory system: the third and fourth weeks
After fertilization and implantation, the gestational sac, a spherical structure that provides nutrients, fills with fluid, and envelops the egg. This allows for rapid growth throughout the first weeks of pregnancy and moves us into the embryonic stage.
During this stage, the external neural tube, which develops into our brain and spinal cord, closes to become an internal structure. Organs like our eyes and inner ears start to take shape. Limb buds, which appear as small stubs, form to mark our future arms and legs. In addition, the digestive and respiratory systems also start to develop, allowing for our intestines and lungs to grow.
Perhaps most notably, around day 16 of conception, the foundation for the circulatory system develops, creating our heart and its blood vessels. This makes the heart the first organ to form in our bodies. At the end of week four of conception, we are one-quarter inch long and the size of a lentil.
Heartbeat: the third and fourth weeks
During weeks five and six of conception, our brain divides into the cerebrum, brainstem, and cerebellum. In addition, small indentations marking our nostrils appear on our now developing nose. At the same time, our lips form, and our eyes and ears become more distinct. The limb buds mentioned earlier now develop into paddle-like structures, allowing our arms, legs, fingers, and toes to take shape.
What people are surprised to hear is that twenty-two days after conception, our heart begins to function and can be seen on an ultrasound. At this time, the heart begins to beat. After week six of conception, we are now half an inch long and the size of a bean.
Major systems: the third and fourth weeks
For weeks seven and eight of conception, our facial features, such as eyes, nose, mouth, and ears become more noticeable. This is the same for our fingers and toes, which are webbed yet distinguishable. Our main systems, including the nervous, digestive, and urinary systems, continue to become more complex. Bones, such as our teeth, jaw, nose, and collarbone, also develop quickly, allowing us to bend our elbows.
At this stage of pregnancy, our heartbeats can be heard using a Doppler. We are also in constant motion and can even hiccup, although the mother cannot feel this. Our major structures and systems also form, and our shape is now recognizable as a human. By the end of week eight, we are between one and one and a half inches long, appearing around the size of a kumquat.
Final developments: the ninth and twelfth weeks
This period marks the beginning of the fetal stage. At this time, the umbilical cord is visible, and our intestines are in the abdomen. In addition, our internal genitalia start fully developing while the external genitalia begin to form (although ultrasound cannot yet determine sex). A more mature larynx also forms, allowing for vocal cord development.
After twelve weeks of conception, we have fully formed (but not fully grown) arms, hands, fingers, legs, feet, toes, earlobes, and eyelids. These various changes allow us to grasp with our hands, open our mouths, sigh, and stretch. Alongside increased movement, our face, hands, and feet can now sense light touches. With our distinguishable fingers, we can also begin developing fingerprints and fingernails. At the end of these twelve weeks, we are now about two and a half inches long and about the size of a plum. For the remainder of the pregnancy, we will continue to develop rapidly, building upon our growth from the first twelve weeks.
How We Can Help
If you believe you’re pregnant and want to verify your pregnancy or progress, we’re here for you. Schedule your free and confidential appointment, or give us a call at 530.272.6800. We would love to meet with you and answer any questions you may have during this time.
Mayo Clinic Staff. “Fetal Development: The 1st Trimester.” Mayo Clinic, Mayo Foundation for Medical Education and Research, 17 Sept. 2021, https://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/in-depth/prenatal-care/art-20045302.
“The First Trimester.” Johns Hopkins Medicine, Johns Hopkins Medicine, 8 Aug. 2021, https://www.hopkinsmedicine.org/health/wellness-and-prevention/the-first-trimester.
Neva Monigatti-Lake, M.D.
The content on this page has been reviewed and approved by our Medical Director.