Baby Development Facts – Defining the Weeks of Pregnancy

Defining the Weeks of Pregnancy

Before I begin describing some of the stages of development, I find it really important to explain what it actually means when a woman is told “You are ___ weeks along in your pregnancy”.    

When I was expecting my babies, this was always very confusing to me:  If a human pregnancy is supposed to last for 9 months, why is a full term pregnancy stated as 40 weeks?  Isn’t 9 months equal to 38 weeks?  The doctor would tell me I was certain amount of weeks along, but I knew that I had not conceived that far back because I kept very detailed records of my cycles and was pretty certain of when I had actually conceived.  Back then, no one took the time to explain this to me.  That is why I’m taking the time to explain it here.

Regarding the “weeks of pregnancy”, there are two terms to understand:

Conception

Gestation

  

Conception occurs at the actual moment that a male sperm and a female egg unite and a new human life begins to form.  This occurs approximately two weeks after the first day of the last menstrual cycle (LMP).  We say “approximately” because every woman is unique and the time when a woman ovulates and releases an egg from her ovary can vary by a few days here and there.  For example, I conceived my daughter 19 days after the first day of my period—almost three weeks after.  From the moment of conception, a full term pregnancy is 38 weeks or 9 months long.  

Gestation is the time period of a pregnancy beginning with the first day of a woman’s last menstrual cycle (LMP).  In other words, a gestation period does not refer to the moment from when a baby was actually conceived.  The gestational period also includes the time during which a woman was last menstruating and ovulating, because during that time her body was preparing for pregnancy.  The menstruating and ovulating time frame lasts approximately two weeks, but can vary from woman to woman.  This is the reason why one of the main questions asked of a woman when she is having a pregnancy test done is “When was the first day of your last menstrual cycle?”  Medical personnel usually refer to pregnancies using the gestational period.   If you have a positive pregnancy test and ask the nurse or doctor how far along you are, they will reply using the gestational time, counting from the first day of your last period.  When speaking of gestation, on the day you actually conceive, you are already two weeks along in your pregnancy.  This is why a full term pregnancy is often referred to as being 40 weeks.

To keep things clear in the Baby Development Facts I post about in the future, I will be referring to the weeks of pregnancy based on the gestation period, as is typically done.  If I refer to a “Day”, it is referring to the day from conception.

Related posts:  Baby Development Facts – Conception and Implantation