Correspondence of weeks of pregnancy to months and trimesters

While being pregnant is an exciting journey, it can be confusing to keep track of all the changes and milestones. While some people refer to months or trimesters, many people discuss pregnancy in terms of weeks. It may be difficult to determine your precise pregnancy stage as a result.

Knowing how the weeks of pregnancy relate to the months and trimesters will help you track your baby’s growth and prepare for what to expect. Additionally, it facilitates communication with friends and medical professionals who might employ various pregnancy measurement techniques.

As you progress through each stage of pregnancy, you can feel confident knowing that weeks, months, and trimesters all line up according to this easy-to-follow guide.

General information

The term "obstetric week" has been introduced to count the term of pregnancy in global obstetric practice. Doctors use them to calculate pregnancy. Thus, when a novice patient arrives for the consultation 7–8 days after the postponement and finds out from the physician that she is in her second month of pregnancy, she is confused and understandably upset—after all, she has only been pregnant for a short time, and they are calling her an extra!

This is where a lot of pregnant women who are certain that "the doctors made a mistake with the term" originate their posts and messages. They were not misinformed. They simply counted in obstetric weeks, and women typically count everything starting on the "day X," which is when sexual activity occurred and may result in the birth of a child.

Only the day of ovulation and the 24 hours that follow are fertile for a woman. However, there are situations when not even the fair sex can pinpoint that exact day. Because this process can be influenced by a number of factors, including stress, colds, and fatigue, the egg may leave the follicle a few days earlier or later. As a result, neither the woman nor the doctor can pinpoint the precise moment of the baby’s conception. The decision was made to measure the period in obstetric weeks as a result. They are counted starting on the first day of the cycle’s final menstrual period.

• Week number one is the time when a woman has menstrual bleeding.
• The second week precedes ovulation.
• If you’re lucky, the fertilized oocyte will implant in the endometrial layer of the uterus on the third month.
• At the end of the fourth week, a delay begins, which makes a woman buy a test and establish the fact of her imminent motherhood.

In other words, the obstetric phase always lasts two weeks longer than the embryonic phase.

An average pregnancy lasts 40 obstetric weeks in total. Now that you are aware of the 14-day difference, it is simple to comprehend that this is 38 embryonic days. A baby’s gestation period lasts between 266 and 280 days on average. This period of time is split up into months and trimesters.

A better understanding of how pregnancy weeks relate to months and trimesters can make it easier for expectant parents to monitor their growing child. Usually, a pregnancy is broken up into three trimesters, each lasting roughly three months. A full-term pregnancy lasts approximately 40 weeks in total, with four to five weeks occurring each month. Knowing how weeks divide into months and trimesters helps parents better track medical milestones and get ready for the changes that lie ahead.

Main terms

A month is thirty or thirty-one days, as we have learned in school. There are 28 or 29 days in February. However, there are differences between the obstetric month and the calendar month.

Obstetric weeks and months

Pregnancy is measured in obstetric months as well as obstetric weeks.

Regardless of the calendar indications from the time of year, each such month has a duration of precisely four weeks.

As a result, there are many similarities between the obstetric and lunar months. Accordingly, every obstetric month lasts exactly 28 days, no more or less:

• first month of pregnancy – 1-4 weeks inclusive (at the end of the month a woman may only find out about her pregnancy);
• second month – 5-8 weeks inclusive;
• third month – 9-12 weeks inclusive;
• fourth month– 13-16 weeks inclusive;
• fifth month– 17-20 weeks inclusive;
• sixth month – 21-24 weeks inclusive;
• seventh month – 25-28 weeks inclusive.
• eighth month – 29-32 weeks inclusive;
• ninth month– 33-36 weeks inclusive;
• tenth month – 37-40 weeks of pregnancy.

Calendar weeks and months

The months of pregnancy will appear somewhat differently on a standard wall or desk calendar:

• first – 1-4.5 weeks;
• second – 4.5-9 weeks;
• third – 9-13 weeks;
• fourth – 14-17.5 weeks;
• fifth – 17.5-22 weeks;
• sixth – 22-26.5 weeks;
• seventh – 26.5-30.5 weeks;
• eighth – 30.5-35 weeks;
• ninth – 36-40.5 weeks.

There will be nothing inexplicable about the tenth month of pregnancy now that you know where it originated.

Obstetric weeks and trimesters

The entire gestational period is separated into three sections for ease of understanding. We refer to each as a trimester. There are three months in a trimester. The trimesters thus appear as follows:

• First trimester – 1-13 weeks;
• Second trimester – 14-26 weeks;
• third trimester – from 27 weeks until birth.

Periods

There are various stages in the fetus’s development from a single cell to a fully formed human organism that will give birth to a newborn. They have a close connection to the obstetric phase as well. There are various time periods:

• Preimplantation. Starts from the first day of the last menstruation (1 week) and lasts up to 21 days, when the fertilized egg can travel through the fallopian tube and attach to the uterine wall (that is, implantation occurs).
• Embryonic. Starts from the moment of fertilization and lasts up to 9-10 obstetric weeks inclusive. The most important period, since at this time all internal organs and systems are laid down and formed. The baby is called an embryo.
• Fetal (foetal). From the 11th week until birth. During this time, the baby grows, all organs are already formed. The baby is called a fetus.
• Perinatal (prenatal) begins from the 23rd week of pregnancy, when the birth of a child will not be considered a miscarriage, but will be classified as premature birth, lasts until the 42nd week.
• Extended begins at 40 obstetric weeks, if labor has not begun.
• Late labor – after the 42nd week of pregnancy.

Table of correspondence of terms (obstetric weeks and months)

Three Months

Weeks

Months

Time Periods

Initially

Embryonic, preimplantation, and fetal

Secondly

Tracking your progress will be easier if you know how pregnancy weeks relate to months and trimesters. Additionally, it helps you understand your body’s and your baby’s development at each stage better.

Three trimesters, each lasting roughly three months, make up a pregnancy. Every trimester has its own turning points and adjustments. Being aware of your pregnancy status aids in getting ready for the future.

It doesn’t matter if you measure in weeks, months, or trimesters—what matters most is continuing to communicate and stay informed with your physician. They’ll support you through every stage and ensure the health of both you and your child.

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Correspondence of weeks of pregnancy to months and trimesters