How Long Is The First Trimester (And Why)?

Exact Answer: 12 Weeks

The first trimester is the first part of pregnancy among the three trimesters. The first trimester begins even before pregnancy, that is to say, it begins from the last day of a woman’s period cycle till the end of the 12th week of the pregnancy.

The first trimester is considered the most crucial trimester among the 3 because this is the time when a fertilized egg undergoes multiple advancements to form an embryo. Moreover, the embryo further develops into a child. Major organs are formed during this trimester and thus considered the most crucial trimester.

Moreover, when this trimester begins, pregnancy is confirmed and females go through many hormonal changes resulting in many behavioral changes as well.

How Long Is The First Trimester

How Long Is The First Trimester?

Generally, and considering situations of healthy pregnancies, the first trimester is for 12 weeks. The first trimester starts from the day when the female egg and male sperm fuse, resulting in the formation of a zygote by the process of fertilization. 

By the end of the first trimester, the physical body of the child is formed. Moreover, major organs like the heart, liver, and arms, legs are also developed. After complete formation of the organ, heart, heartbeats can be heard of the child by using lab techniques. Doctors examine the heartbeats of a child and give you further reports about the health of the child.

However, in some cases of unhealthy pregnancies or complicated pregnancies, the first trimester can be delayed up to 15 weeks also. There could be many reasons for the first trimester to get extended. For situations like mother doing more physical exercises, lack of nutrition, unhealthy lifestyle, lack of sleep and rest, and other such situations can lead to a change in the first trimester.

Moreover, if the mother has medical complications, any disease which can affect the child and pregnancy or is simply unhealthy, then also there are chances when the pregnancy can get complicated and this may affect the period of first pregnancy and make it extend.

Type Of First TrimesterTime
Normal first trimester12 weeks
Prolonged first trimester13 to 15 weeks

The first trimester being the most crucial, it is important to be extra precautious during this time. It is important to take proper supplements and medications as prescribed by the gynecologist, maintain a healthy diet and sleep schedule, and live a healthy lifestyle. If all the measures are followed then pregnancy definitely will go well and the first trimester will last for 12 weeks only.

Why Is The First Trimester That Long?

During the first trimester, many events take place inside the woman’s body and in the zygote. Multiple advancements altogether take up to 12 weeks. Thus the first trimester takes 12 weeks.

The first event that occurs is fertilization. That means, formation of a zygote. The zygote further gets implanted into the uterine wall of the female and develops into an embryo. This whole process of formation of the zygote to embryo takes around 1 to 2 weeks.

Once the embryo is completely formed, it starts developing into a child at a fast rate. It is scientifically said that the growth and development of a child are fastest during the first trimester.

Moreover, the first organ that gets developed is the heart. It takes up to 4 weeks for complete formation of the heart and by the end of 6 weeks, the child’s heartbeats can be heard. Consequently, organs like the liver, kidney, and lungs are formed along with the formation of the heart. 

Apart from this, the child’s sex organ also gets developed and by the end of the first trimester, the child’s sex can be determined. Moreover, the child’s muscles and bones are also developed which leads to the formation of arms and legs.

All this process takes up to 12 weeks and thus, that’s why the first trimester lasts for 12 weeks.


By the end of the first trimester, the major body and major body parts of the child are formed. During the rest of the two semesters, the child just undergoes further development. That is why the first trimester is considered the most crucial one.

Scientifically it is said that by the end of 12 weeks, the child grows around 3 inches in length and can weigh up to one ounce. However, it depends and varies from pregnancy to pregnancy.

Thus, it is important to be careful during the first trimester and take proper care of not just the child, but the mother as well.



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Hi! I'm Nidhi.

Here at the EHL, it's all about delicious, easy recipes for casual entertaining. So come and join me at the beach, relax and enjoy the food.

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