Conception dates

Trimester Calculator

How do you calculate trimesters?

Pregnancy Trimester Calculator


Trimester Calculator

Trimester Calculations from LMP, Conception or Retrieval dates.
  • In an IVF cycle, Ovulation is equivalent to Retrieval and this calculator will subtract two weeks from this date to generate the LMP .

  • To Calculate your LMP date, enter your Due Date in the Conception Calculator
This page is consistently ranked first on Google for the keywords "Trimester Calculator"
Enter your Reference Date, then click the Calculate Trimesters button.
Reference Date:     LMP
Ovulation / IVF


Possible Dates
of Conception:
You are now:
Trimester dates
calculated by:

Development  ?

Gestation  ?

Conception  ?
Second Trimester Begins
Third Trimester Begins
The three methods of calculating trimesters by:
Development, Gestation, Conception are explained below.
Due Date:   Single

Multiples - Due Dates by Average Gestation:  ?
No matter what method is used; a pregnancy has a beginning, middle and end. You can use the trimester dates from your Doctor, a book or a website, whichever you feel most comfortable with.
  When or what week does the first trimester end or second and third trimester begin
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The three basic ways to calculate trimesters

There are basically three ways of dividing up a pregnancy into the three trimesters; they are by:
Development,   Gestation,   Conception. They give different dates for when the second trimester begins and for when the third trimester begins. Your health care provider might prefer to use the Development method, while another Midwife or Doctor may use the Gestation method.
I am sure they do that just to confuse pregnant women.  Big Grin

by Development:

This uses actual developmental stages to divide up a pregnancy. From LMP to 12 weeks the embryo develops all the major organs and becomes a fetus. From 12 weeks to 27 weeks the fetus continues developing and reaches viability. From 27 weeks on the fetus finishes development and prepares for delivery. Here the second trimester begins at 12w 0d and the third at 27w 0d.

by Gestation:

With this method you take the 40 weeks of gestation and divide into three equal stages. Here the second trimester begins at 13w 3d and the third at 26w 6d.

by Conception:

This method is where you take the 38 weeks of post conception development, divide by three, into the three equal trimesters. Here the second trimester begins at 14w 5d and the third at 27w 3d.

What is the length of the first, second and third trimesters?

The length of the trimesters is often a source of confusion. The word trimester means 'three months'. Three periods of three months each, gives pregnancy a beginning, a middle and an end period.

These periods match the developmental stages to divide up a pregnancy.

  • Beginning: From LMP to 12 weeks the embryo develops all the major organs, becomes a fetus, and the placenta takes over control.
  • Middle: From 12 weeks to 27 weeks the fetus continues developing and reaches viability.
  • End: From 27 weeks on the fetus finishes development and prepares for delivery.

When does the first trimester end or third trimester begin?

Does it really matter exactly when you are out of the first trimester or start the third trimester? No. When the trimesters start and end depends on who you ask, which book you read or what website you visit. Most health care providers actually talk about your pregnancy in weeks, rather than months or trimesters.
Return to the Trimester Calculator


Fetal Development

Now you know your trimester dates, find out how your baby develops over the weeks and months while inside. The Fetal Development page gives you a breakdown of just what is happening and when, to baby and you in your pregnancy.

Disclaimer: The information provided in this section and this website is for the entertainment of our visitors and is only for educational purposes. It is not intended to take the place of the advice and/or care of a qualified healthcare professional. Remember that every woman and every pregnancy is unique, and provides only general information about conception, pregnancy and birth.

The information contained in this site should not be used as a substitute for care you should receive from a qualified healthcare professional. If you have questions or concerns about your health or the health of your baby, please seek the advice of your healthcare provider.



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