Schedule Today

First Four Weeks After Conception

Pregnant. We understand that for most of our clients, this is not good news. This is not a time or situation when you expected or wanted to become pregnant. Questions are running through your mind: “What is happening to me?” “What does this mean?” “What do I do next?” “What are my options?”

If you are wanting to know what happens in the first four weeks after conception, we hope this article will help you with the information you are seeking. If you need to explore your options, we hope you’ll take another step and contact us at Avenue Women’s Center. We are here to share options and give assistance to women who are challenged by an ill-timed or unwanted pregnancy. It has been our purpose for over thirty-five years in the Chicagoland area to come beside those who need nonjudgmental care and support for this difficult time. We hope you will allow us to take this journey with you. You can reach us by phone, text, email or chat. We’re waiting to hear from you.

The first four weeks

Several different methods are used to follow the time course of events after conception of a human embryo. It is not normally possible in practice to identify the exact time of conception; therefore, clinicians typically count from the time of a woman’s last menstrual period. That approach can often lead to a time as much as two weeks different from the actual time the egg, or ovum, was fertilized. An embryologist, however, will count from the actual time of conception. For this article, we will use the embryologist’s timing.

When a human ovum is released from the ovary, it is just a fraction of a millimeter in diameter—no larger than the period at the end of this sentence. It has just undergone a “reduction division” in which the number of chromosomes in its nucleus have been reduced by half and is available to be fertilized by a sperm cell. At the time it is fertilized it is traveling down the oviduct toward the uterus. That trip typically requires from six to ten days. During that time, initial cell divisions begin and by the time it reaches its destination, it has formed itself into a multi-cellular structure (a few hundred cells) called a blastocyst—still only slightly larger than the fertilized egg. This is the stage in which the embryo implants in the wall of the uterus. Implantation, however, is not a guaranteed event. Almost half of all conceptions do not implant and the blastocyst passes through the uterus and is discarded.

From the time of implantation until life birth, development of the embryo will progress through a specific set of stages and a time course that never changes. Following is a summary of those stages for the next three weeks of development:

During week two, the blastocyst continues to imbed itself more intimately into the wall of the uterus. Its cells begin to migrate within the embryo, influencing one another in ways that cause them to differentiate into more specialized types of cells. These cells ultimately aggregate in certain areas of the embryo as one of three tissue types: 1) ectoderm, which will become primarily skin and nervous tissue; 2) mesoderm, which will become primarily muscle and bone; and 3) endoderm, which will become the digestive system.

During the third week, differentiation of the embryonic cells continues. The uterine tissue around it continues to form what will become the placenta. A longitudinal axis will become well established, and the beginnings of a central nervous system will be apparent. The mesodermal cells will have established the beginnings of a cardiovascular system. The size of the embryo is now perhaps 2 mm in length

In week four, differentiation and specification continue. Heart chambers have formed by this time. Other internal organs and limb buds are beginning to appear. The embryo is now perhaps 5 mm in length—still microscopic if you expect to observe any detail.

So there you have some information about the first four weeks after conception. As we noted earlier, if the questions you are facing go deeper than what is happening physically with this pregnancy, we urge you to call, text, email, or chat now with a caring professional.

At Avenue Women’s Center, we strive to meet our clients wherever they are at in their decision-making process, whatever their personal questions may be. Answers and support are available in the context of free, private, confidential pregnancy consultations. If you are interested to learn more or schedule your appointment, please contact us today.


  • Embryology. (2016, August). Embryonic Development. Retrieved from:
  • Embryology. (2017, September). Timeline Human Development. Retrieved from:
  • American Congress of Obstetricians and Gynecologists (ACOG). (2015, June). Prenatal Development: How Your Baby Grows During Pregnancy. Retrieved from:
  • The Free Dictionary. Blastocyst. Retrieved from:
  • The Free Dictionary. Germ layer. Retrieved from:

Reviewed by Patricia Kuenzi, APN-CNP, MSN, ANP, PNP.

The information provided here is general in nature.  It is not a substitute for a consultation with a medical professional. Before any medical procedure, it is imperative that you discuss your personal medical history, risks, and concerns with your doctor. If you have questions during or after a procedure, your doctor should be immediately contacted. Avenue Women’s Center is not an emergency center.  If you are experiencing severe symptoms, such as bleeding and/or pain, seek immediate medical attention.  Contact your physician, go to an emergency room, or call 911.