Weeks 3 & 4 of Pregnancy: The Beginning of New Life
September 10, 2025
Pregnancy is an incredible journey, marked by rapid changes and milestones that begin long before many women even realize they are pregnant. Weeks 3 and 4 are critical stages when new life takes root and begins to develop at a remarkable pace.
At Amnion, we know these earliest weeks can bring many questions and emotions. Whether you’re excited, anxious, or simply curious, we want to provide accurate information and compassionate care as you take your next steps.
Week 3: Fertilization and Early Development
Around week 3, fertilization takes place. This occurs when a sperm cell successfully meets and penetrates an egg cell, forming a single-celled organism called a zygote. The zygote carries a complete set of genetic instructions – DNA – from both parents, which determines traits such as hair color, eye color, and even aspects of personality.¹
As the zygote travels down the fallopian tube toward the uterus, the cells begin dividing rapidly. By the time it reaches the uterus, usually 3-5 days after fertilization, it has become a cluster of cells known as a blastocyst.²
The blastocyst has two main parts:
- The inner cell mass, which will develop into the embryo.
- The outer layer of cells, which will become the placenta, providing nutrients and oxygen to the growing baby.
Week 4: Implantation and Embryo Formation
By the beginning of week 4, the blastocyst has implanted into the lining of the uterus. This step is essential for pregnancy to continue. Once implantation is complete, pregnancy hormones begin to rise, which is why many women can now receive a positive pregnancy test result.³
At this stage, the developing zygote is referred to as an embryo. Although still smaller than a poppy seed, the foundations of vital organs are already forming:
- Neural tube development: The structure that will become the brain and spinal cord begins to form.
- Heart development: The heart starts to take shape and will begin to beat in the coming weeks.
- Placenta and amniotic sac: These structures develop to provide protection and nourishment.⁴
Even though outward signs of pregnancy may still be minimal, incredible changes are happening at the cellular level that set the stage for all future growth.
What You May Experience
During weeks 3 and 4, some women notice signs of early pregnancy, while others feel no different. Common experiences may include:
- Mild cramping or spotting, sometimes related to implantation.
- Breast tenderness or swelling due to hormonal changes.
- Fatigue as the body begins producing more progesterone.
- Missed period, which is often the first clear sign that leads to pregnancy testing.⁵
Remember: Every woman’s body is unique and symptoms can vary widely.
Amnion is Here for You
Learning you may be pregnant can feel overwhelming, but you don’t have to walk this out alone. What we offer:
- FREE & confidential pregnancy testing
- Ultrasounds to confirm pregnancy and provide vital information
- Options counseling to talk through your questions and choices with compassion
- Ongoing support services to walk with you every step of the way
No matter what you’re navigating, we believe every beginning matters and we are here to support you.
Ready to Talk?
Whether you’re weighing your options or processing your past, we’re here for you.
📞 Call us at 610.622.9957
📧 Email: info@amnion.org
📍 Or schedule a free appointment on our website: amnionpregnancycenter.com
Let us walk with you through this season, because you were made with purpose, and you are not alone.
Sources:
- Moore, K. L., Persaud, T. V. N., & Torchia, M. G. (2020). The Developing Human: Clinically Oriented Embryology (11th ed.). Elsevier.
- American College of Obstetricians and Gynecologists. (2023). Stages of Pregnancy. https://www.acog.org/womens-health
- Mayo Clinic. (2023). Fetal development: The 1st trimester. https://www.mayoclinic.org
- Sadler, T. W. (2018). Langman’s Medical Embryology (14th ed.). Wolters Kluwer.
- Cleveland Clinic. (2023). Early Signs of Pregnancy. https://my.clevelandclinic.org