November 21, 2014
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Defining the trimesters of pregnancy

What is the origin of the word trimester? Like many English words, the word "trimester" has Latin origins. In this instance, trimester comes from the Latin word trimestris, which means tri (three) and mensis (month). Thus, trimester is technically defined as a 3-month period that has origins related to menses and menstruation.

Unfortunately, not all months are created equal, and 3 months per pregnancy trimester is just a general rule of thumb. There are several interpretations of the word trimester in regards to pregnancy, and the period of time defined is related to preconception, conception, and fetal development. Although they will provide different dates as to when one trimester ends and another begins, they all equate to approximately similar timelines of 3-month periods. Regardless of the method used, a pregnancy has a beginning, middle, and end - known as first, second, and third trimesters.

Preconception (gestation) trimesters

The majority of health care providers determine trimesters under the guideline that a typical pregnancy lasts 40 weeks (280 days) starting from the first day of your last menstrual period. Using this method, each trimester is determined by dividing 40 weeks by 3.

Technically speaking, the first trimester is 13.33 weeks, which is approximately 13 weeks and 2 days. The second trimester begins at 13 weeks and 3 days and ends at 26 weeks and 5 days, and the third trimester begins at 26 weeks and 6 days. For the sake of simplicity, each trimester is rounded down to the nearest week.

Trimester Month Week
First
(Weeks 1-13)
1
1-4
2
5-8
3
9-13
Second
(Weeks 14-26)
4
14-17
5
18-21
6
22-26
Third
(Weeks 27 until birth)
7
27-30
8
31-35
9
36-40

You can also use this 40-week/280-day rule to figure out your approximate due date from knowing the date of when your last menstruation started. For example, if you know the first day of your last period was on October 17, then you can count 40 weeks ahead from that day to get your approximate due date.

Other methods that are used are based on fetal development and knowing your date of conception.

Conception trimesters

This method is rarely used because of the difficulty in determining the actual date of conception (egg fertilization), unless in vitro fertilization was used, Most women only know when their last menstruation started and that is why most doctors will use the preconception method. Nonetheless, this method uses the guideline that a baby is full-term 38 weeks after conception, coupled with the fact that conception usually occurs 14 to 16 days after the first day of your last period. Thus, trimesters are calculated using this method by dividing 38 weeks by 3 and then adding 2 weeks for time of conception. Using this method, the second trimester begins at 14 weeks and 5 days and the third at 27 weeks and 3 days. Again, this method is rarely used even though the trimester timeline are similar to the commonly used preconception method.

Development trimesters

This method uses a biological basis for defining trimesters via developmental stages of your baby. These distinct fetal developmental periods last approximately 3 months each and correlate well with other interpretations of pregnancy trimesters. The first trimester is a miraculous period in which the single-celled embryo multiplies rapidly, developing into a fetus with all major organ systems by the end of 12 weeks from the start of you last menstrual period. The second trimester is no less miraculous, marked by rapid growth and maturation of body systems until the fetus reaches a point where it has a good chance of surviving outside of the womb at the end of Week 26. The third trimester, beginning at Week 27, is the time in which the fetus finishes developing and becomes well prepared for life outside of the womb.


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