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How to calculate your pregnancy in weeks and months

There are about 40 weeks in a pregnancy, starting from the first day of the prospective mother's last menstrual period. Pregnancy is usually calculated as per the date of the Last Menstrual Period (LMP).

How to calculate your pregnancy in weeks and months
There are about 40 weeks in a pregnancy. (Source: Getty Images)

By Dr Rita Bakshi

Many pregnant women are often muddled about getting the right count of the exact number or months of their pregnancy. Here are answers to a few common questions on calculating one’s pregnancy:

How many weeks pregnant are you?

There are about 40 weeks in a pregnancy, starting from the first day of the prospective mother’s last menstrual period. Pregnancy is usually calculated as per the date of the Last Menstrual Period (LMP). To calculate how many weeks pregnant the prospective mother is, ask her when she last had her period. The first day of her LMP is counted as the first day of the pregnancy.

How to calculate your pregnancy in weeks and months
Pregnancy is usually calculated as per the date of the Last Menstrual Period (LMP). (Source: Getty Images)

What if you don’t know the date of your last period or have irregular periods?

The due date calculation works best if the menstrual cycle is regular and the periods take place every 28 days. Just in case, one has an irregular menstrual cycle, then she might be unable to figure out her date of LMP. Or if her cycle differs in length, calculating from the first day of her LMP might not give us a date that we can trust. However, it can be really tough figuring out the date of the last period if the prospective mother has irregular period.

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But the dating scan that is the first ultrasound of the prospective mother accurately tells us about the due date of the baby. The sonographer will carry out Crown Rump Length which will give an estimate idea of how far along is the pregnancy.

Also Read: Are home pregnancy test results always accurate?

What is a trimester?

A pregnancy is divided into trimesters, of three months each. Pregnancy usually lasts around nine months and each pregnancy has about three trimesters. Each trimester has its own set of changes that the body and the baby undergoes.

The first trimester lasts from the very beginning of the pregnancy until the mother is 13 weeks plus six days pregnant. The second trimester lasts from when the mother is 14 weeks to 27 weeks plus six days pregnant. The third trimester lasts from when the mother is 28 weeks pregnant until she has her baby.

In the first trimester, the prospective mother is usually exhilarated about being pregnant and starts taking precautions. She needs to make some lifestyle changes. During the second trimester, the prospective mother feels much better as she is accustomed to the changes that her body is experiencing. She may have to deal with problems like morning sickness. In the third trimester, the mother needs extra energy to cope with the changes and might feel tired, undergo mood swings and experience fatigue.

How long does pregnancy last – is it 36 weeks or 40 weeks? What is the ideal period and how is it determined?

The tenure of the pregnancy is usually for about 40 weeks. Most women count 40 weeks from the first day of their LMP. It usually varies in calendar months and lunar months.

The normal length of a pregnancy can be anything from 37 to 42 weeks. A baby can arrive at any time between that. If the mother gives birth before 37 weeks of her pregnancy then the baby could be premature. Also, premature babies require extra care, nourishment and attention.

How accurate is my due date?

The due date is a guesstimate of when the prospective mother will give birth. It is just an estimation and it is not necessary that the birth of your child takes place exactly on the same date as the due date. It is okay for a baby to arrive between 37 weeks and 42 weeks of pregnancy.

(The writer is a senior gynaecologist and IVF expert, International Fertility Centre.)