For the purpose of calculating your due date, pregnancy starts on the first day of your last menstrual period (LMP). Using this start date, an average pregnancy lasts about 280 days. That’s easy and simple and that is how Pregistry’s Due Date Calculator calculates all your important dates.  However, if you define the real start of pregnancy as the time when a new life begins, the start date is not so simple. [2,3]
If pregnancy really begins when life begins, there are two possible, scientific answers. Some experts say pregnancy begins at the time of conception, when a sperm fertilizes an egg. Some experts say pregnancy begins when a fertilized egg implants into the lining of the uterus. If you throw in the less scientific opinions of religious scholars, political activists, and lawmakers, you fall into a debate that may never be settled. [2,3]
The Basics of Conception and Implantation
It was not until 1839 that science recognized a single cell – the zygote – as the beginning of life.  A zygote is the cell formed when a sperm fertilizes an egg. The common term for this event is conception. When an egg is released during ovulation, a sperm may fertilize it. When this happens, the single-celled zygote is created. [2,4]
Conception occurs in the fallopian tube. The zygote begins the process of cell division as it travels down the fallopian tube to the uterus. This trip can take about 5 to 8 days. The zygote now has several cells and has become a blastocyst. The blastocyst may implant in the lining of the uterus at about 9 to 10 days. Once implantation takes place, the blastocyst can develop into an embryo. The embryo continues to develop until at 8 weeks after conception, it is considered a fetus.  So when does pregnancy really start?
Pregnancy Starts at Conception
Once a sperm penetrates an egg, changes take place that may lead to the formation of a complete human being, a new life. This is not debatable from a strictly embryologic perspective. The zygote immediately starts to make changes that will lead to replication of DNA, cell division, and formation of a fetus. The single-cell zygote is very different from the sperm or the egg. It is a potential person. 
The belief that conception is the beginning of pregnancy is strongly stated by the American College of Pediatricians. This group is a conservative, advocacy group, (not the same as the American Academy of Pediatricians), but they are not alone in this view. In fact, according to a survey reported in the American Journal of Obstetrics & Gynecology, most OB-GYN doctors also believe that pregnancy begins at conception. This survey asked over 1,000 American OB-GYN doctors when pregnancy begins: 
- Fifty-seven percent of the doctors said pregnancy begins with conception.
- Twenty-eight percent said pregnancy begins with implantation.
- Sixteen percent said they were not sure.
Pregnancy Begins at Implantation
This is the official view of the American College of Obstetricians and Gynecologists (ACOG), which is the recognized association for OB-GYN specialists in the United States. According to this view, implantation is the key event that allows a zygote to develop into an embryo and a fetus. Without implantation, there is no pregnancy. Up to one-half of fertilized eggs never implant in the uterus. [2,5]
The view that pregnancy begins at implantation has long been the position of the U.S. Federal Government. This is important because if federal policy changes to conception as the start of pregnancy, intrauterine devices (IUDs) and emergency contraception pills (morning after pills) would change from contraception to abortion. They would not be preventing pregnancy, they would be terminating pregnancy. Because of the Hyde Amendment – passed in 1976 – Medicaid is not allowed to pay for abortion. If conception becomes the rule, the federal government could no longer pay for these services for poor women. 
The Future of the Debate
Where you stand on this issue may depend on your social, religious, or political beliefs. The survey in the American Journal of Obstetrics & Gynecology confirms several previous surveys of American physicians that found about a 50/50 split between conception and implantation as the start of pregnancy. A doctor’s own religious beliefs play a major role on where they fall in this debate. 
Many states have jumped into the debate and passed laws that support one side or the other. In Alabama, pregnancy begins at conception. In California, pregnancy begins at implantation. Unfortunately, many of these laws have little to do with science. In some states, terms like conception, implantation, and pregnancy are used interchangeably. It becomes a purely religious or political issue. 
This debate will go on. Policies may change. For many women, it is enough to use the last day of the last normal period as the official start of pregnancy. The debate about when life starts has been going on for centuries. The miracle of new life defies easy explanation. For many women pregnancy really starts when they have a positive pregnancy test, they can see their baby on an ultrasound, and they can feel their body start to change.
- American Pregnancy Association, Pregnancy Week 1 & 2.
- Guttmacher Policy Review, The Implications of Defining When a Woman Is Pregnant.
- American College of Pediatricians, When Human Life Begins.
- Merck Manual, Stages of Development of the Fetus.
- American Journal of Obstetrics & Gynecology, Obstetrician-gynecologists’ beliefs about when pregnancy begins.