Baby Developing inside you

Weeks, Months and Trimesters

Baby developing inside you
over the weeks and months
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Weeks, Months and Trimesters Guide

A normal pregnancy lasts 280 days from your LMP which is about 266 days from conception to birth. The first two weeks of your pregnancy includes the time of your period, ovulation and fertilization. Conception occurs approximately 14 - 16 days after the first day of your period.

The date of conception is not always known and can vary between women. The date of LMP is known by most women so doctors use this date as a starting point for pregnancy.


  • 40 weeks from the start of your LMP (Gestational).
  • 38 weeks from conception to birth (Fetal age).
Most doctors and health care providers count pregnancy in weeks from LMP (Gestational), a few others may count from conception to birth (Fetal age); and the week numbers may vary.
Example:  Week 28 (Gestational)  =  Week 26 (Fetal age)

To find out how many weeks pregnant you are and what is going on in your bump.
Week by Week Fetus Development


  • Measured as nine months on the calendar -- starting one week after your LMP.
Example:  LMP - January 1st  =  Due Date - October 8th

Lunar Months: When people claim that a pregnancy is exactly 10 lunar months, they are making the mistake of calculating a lunar month to be 28 days long. Lunar months approximate to 29.53059 days (29 days, 12 hours, 44 minutes and 3 seconds). A normal average pregnancy (280 days) is approximately nine and half lunar months, since a lunar month is approximately 29.53 days long.

Most doctors and health care providers count pregnancy in weeks and not months, but friends and family will often ask "How many months". Do not make the mistake of working it out as 4 weeks = 1 month, use our weeks to months convertor.


  • The word trimester means 'three months'.
    From Latin trimestris from tri- ('three') + mestris ('month')

  • Months 1, 2 and 3 are the first trimester, months 4, 5 and 6 are the second trimester, and months 7, 8 and 9 are the third trimester.

  • Most commonly, the first trimester is defined as from LMP through to end week 13.
    The second trimester is from week 14 through to end week 26.
    And the third trimester is from week 27 through to labor/delivery, which varies considerably but averages at week 40.
    13weeks + 13weeks + 13weeks = 39weeks   + 1week = 40weeks
Most health care providers will actually talk about your pregnancy in weeks, rather than months or trimesters. Nevertheless, some health care providers and pregnant women talk about the three trimesters to help give pregnancy a beginning, middle, and end.

Most pregnant women worry about miscarriage in the beginning, during this period of time all organ development takes place so the fetus is most susceptible to damage from toxins, drugs, and infections. The end of the first trimester is often seen as a 'mile stone' for fetus survival. The second trimester is the 'easy' middle part. At the end of the third trimester is when the hard work starts...
When do you start your second or third trimester?
Find out with our Trimester Calculator


More Information:


More information on the
First Trimester  |  Second Trimester  |  Third Trimester



Week By Week Pregnancy Calendar

First Trimester
Week 1   Week 2   Week 3   Week 4   Week 5   Week 6   Week 7  
Week 8   Week 9   Week 10   Week 11   Week 12   Week 13
Second Trimester
Week 14   Week 15   Week 16   Week 17   Week 18   Week 19   Week 20  
Week 21   Week 22   Week 23   Week 24   Week 25   Week 26
Third Trimester
Week 27   Week 28   Week 29   Week 30   Week 31   Week 32   Week 33  
Week 34   Week 35   Week 36   Week 37   Week 38   Week 39   Week 40   Week 41   Week 42


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