When does pregnancy begin?

John K. Jain, MD
OBGYN (Obstetrics & Gynecology)

In nature, the human egg only lives 24 hours after ovulation. It is during this time that sperm must fertilize the egg. Approximately 1 week later, 7-8 days after ovulation and 21-22 days after the beginning of the menstrual period, the embryo (fertilized egg) implants into the inner lining of the uterus called the endometrium and quickly sets up a connection with maternal blood vessels. As the pregnancy grows it releases the pregnancy hormone called human chorionic gonadotropin (hCG) into the maternal blood. This hormone can be detected one week after implantation (28 days from the beginning of the menstrual period and 2 weeks after ovulation) in the blood or urine. So, pregnancy officially begins at the time of implantation but is usually not detectable until the pregnancy hormone shows up in the blood and urine.

Diana K. Blythe, MD

When a pregnancy begins is actually somewhat complicated. Many people think that the pregnancy starts when a sperm fertilizes and egg. This is hard to argue, but doctors tend to say that pregnancy begins when the fertilized egg implants in the wall of the uterus. It really depends on whether you think pregnancy starts at fertilization or implantation.

To make it more complicated, pregnancy dates begin to be counted on the first day of a women's last period. This is before ovulation, before fertilization and before implantation. 

Paula Greer
Midwifery Nursing

Most people think pregnancy begins when the female egg and the male sperm make that connection. In the medical field we actually define the pregnancy as beginning when the fertilized egg actually implants or beds itself into the uterus. This corresponds with the first missed menstrual cycle. Pregnancy is actually 10 menstrual cycles. For short cycles that maybe the baby that delivers at 37-38 weeks. For the normal menstrual cycle that may be 39-40 weeks and for prolonged cycles that may be the baby that wants to stay in till 41 weeks.

According to the American College of Obstetricians and Gynecologist (ACOG) Pregnancy is established when a fertilized egg has been implanted in the wall of a woman's uterus.  This corresponds to the point in time when the female menstrual cycle is interrupted.  When the fertilized egg implants the future placental tissues secrete chorionic gonadotropin which maintains the corpus luteum which prevents shedding of the uterine lining or menstruation.  Thus, a pregnancy is considered to be established only when the process of implantation is complete.

This definition is based on the statement of ACOG.  However, the question of when pregnancy begins is an area of potential controversy.  Thus, it is important to recognize that cultural, religious and personal belief systems may influence individual and community based interpretations of when pregnancy begins.

Dr. Mehmet Oz, MD
Cardiology (Cardiovascular Disease)
Pregnancy begins when a mature egg from a woman is fertilized by a mature sperm from a man. Learn more about how this happens in this animation narrated by Dr. Oz.


Pregnancy begins when a woman's ovum is fertilized by a male's sperm. However, often doctors "date" a pregnancy from the first day of the woman's last period. This method of dating a pregnancy results in a slight discrepancy (about two weeks, depending on when fertilization occurred) between the pregnancy in weeks and in the actual age of the baby. For example, a woman may be six weeks along in her pregnancy, but the baby may be only about four weeks old.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.