When does pregnancy begin?
In this series of guest blogs, Dr Grace Petkovic takes us through some of the basics of reproduction and reproductive health.
Grace is a junior doctor in Liverpool, England, with an interest in reproductive endrocrinology.
When does pregnancy begin?
In the last blog: What is fertilisation? We learned that fertilisation describes the process when the egg fuses with the sperm to form a new and unique human organism.
However, doctors count the weeks of a woman’s pregnancy from a different start date.
What start date do doctors use for pregnancy?
Doctors calculate length of pregnancy from the first day of the mother’s last menstrual period (LMP).
Fertilisation occurs on day 14 of a woman’s menstrual cycle, her period will not come when it normally would on day 28. If she immediately tells her doctor, the doctor would estimate that she is 4 weeks pregnant.
Wait a minute! Her egg was only fertilised on day 14! 2 weeks ago! Why is the doctor telling her she is 4 weeks pregnant?
Because doctors start counting pregnancy from the first day of the LMP.
Why do doctors count pregnancy from the LMP?
Because most women do not track their menstrual cycles so they don’t know exactly when they ovulate, but most women do track their periods. So doctors use periods as a marker because most women remember having them.
Even though doctors calculate the weeks of pregnancy from LMP, they may define pregnancy as starting at a different point.
The Royal College of Obstetrician & Gynaecology (RCOG) specialists in women’s health, define pregnancy as beginning at implantation. Once implantation is completed, the RCOG says pregnancy is established.
Why count pregnancy from implantation?
Possibly, because some zygotes (the pre-embryo) do not successfully implant.
Remember the Zygote is a term used for the new organism from fertilisation until 2 weeks after fertilisation, before implantation. This zygote needs to attach itself (‘implant’) into the lining of the uterus. When it is attached to the uterus, the term used is embryo.
Some estimates suggest between 1/3 and ½ of zygotes do not successfully implant.
Another possibility could be because over-the-counter pregnancy tests measure a hormone called ‘bHCG’. This is produced by the embryo in larger quantities after implantation. It’s the embryo’s signal ‘I’ve arrived!’. So that’s often when we can measure pregnancy starting. Remember that that organism is present in the mother, even before we can ‘measure’ it. There may be a difference between when we can measure something starting and when it actually starts.
Confused?
You are not alone! A survey of obstetrician-gynaecologists in the US in 2010 found that over half (57%) would define pregnancy at conception. Fewer (28%) defined it, as the College does, at the point of implantation [1], 16% were not sure [2].
In summary, doctors, patients and scientists sometimes use the word ‘pregnancy’ to mean different things. To clarify:
‘Pregnancy’ can be used to mean when we start to calculate its length
‘Pregnancy’ can be used to mean when the egg has been fertilised by the sperm
‘Pregnancy’ can be used to mean when implantation happens
Read the next blog: ‘How do contraceptives act?’ to learn why these terms matter.
References
[1] The American College of Obstetrician and Gynaecologists defines pregnancy in the same way as the Royal College of Obstetrician and Gynaecologists
[2] Chung GS, Lawrence RE, Rasinski KA, et al. Obstetrician-gynecologists' beliefs about when pregnancy begins. Am J Obstet Gynecol 2012;206:132.e1-7.