What happens in each Pregnancy Trimester?

Pregnant patient in third trimester of pregnancy

Pregnancy can be divided into three time periods. A pregnancy trimester is roughly three months, making up your 9 months of pregnancy.

  • The first trimester is between 0-13 weeks
  • The second trimester we can call 14-27 weeks
  • The third trimester we refer to 28-40 weeks.

These divisions are quite arbitrary, but you will hear of those terms and read about those terms quite often.

Note that I actually have the 14 weeks long second trimester because pregnancy is actually 9 months and 1 week – 40 weeks. Just slightly longer than 9 months.

Pregnancy Dating

Dating of pregnancy can be a little bit confusing, because as many patients correctly point out to me, it is actually only 38 weeks from the time of fertilisation or conception of the pregnancy. However, for hundreds of years historically, pregnancy is conventionally dated from the beginning of the last menstrual period, which usually is around 2 weeks prior to ovulation. Hence pregnancy being approximately 40 weeks long on average.

First Trimester

In the first trimester, the sorts of things that happen are the discovery of pregnancy itself. Usually around 4-5 weeks after the last period, and the development of a lot of pregnancy symptoms which are discussed in this video on the signs of early pregnancy, such things as nausea, tiredness, breast tenderness, bloating of the tummy, change in bowel habit, noticing a need to empty the bladder more often and being more thirsty, are just a few of the typical normal symptoms that a woman will encounter during early pregnancy.

Generally there is a visit to the doctor and some tests. Those tests might include an ultrasound to date the pregnancy, and a variety of blood tests such as your blood count, checking your blood group and antibody status, making sure you are immune to certain viruses and negative for other viruses, also checking thyroid, vitamin D and perhaps other things if you bring any medical history to the table. Also, there will be a discussion about options around pregnancy screening.

Unfortunately, also one of the things that might happen in the first trimester, is the failure of a pregnancy to progress i.e. from miscarriage.

Second trimester

The second trimester tends to be a time when women feel quite well. You will get your second wind around 15-16 weeks and usually things like nausea and vomiting, mostly tends to lift and disappear and you get your energy back and many women actually get to start to enjoy being pregnant a lot more. One of the things that makes you enjoy your pregnancy more is that as the second trimester progresses, you should be feeling your baby move. Usually you will start to feel the baby move at about 18 and 22 weeks, your foetal movements can depend on whether the placenta is at the front or the back of the uterus – this will often determine how early or late you feel any movements. It’s not a competition! There is always someone you will meet who says they can feel the baby at 16 weeks. Don’t worry if you can’t feel it before 20 weeks – it’s probably because the placenta is sitting at the front.

During your second trimester you will be sent for an ultrasound of around 20-22 weeks to have a look in detail at the internal anatomy of the baby. Looking for any structure problems, obviously hoping it is all going to be normal. Towards the end of the second trimester, there will be a glucose tolerance test to screen for diabetes in pregnancy.

Third Trimester

Finally, the third trimester. That’s the time you will be starting to get quite big between 30 and 40 weeks, the baby is really putting on a lot of weight, and you will start to feel physically more uncomfortable, sleep might get a little bit more disturbed, and you will be needing to turn from side to side, waking up very often to go to the toilet to empty your bladder. Foetal movements should continue right through that time, they may reduce a little bit near the end. But any significant reduction in foetal movement should be reported to your doctor, midwife or the hospital you are attending.

Pregnancy check ups are typically more frequent in your third trimester. They go fortnightly and then towards the end of pregnancy, I see patients ever week, often discussion then turns to be about your delivery, concerns you might have around labour, understanding a normal labour process, understanding more about caesarean section if that’s the plan you have, and recognising that you can’t exactly plan every moment of your labour. Having an open discussion with your medical care team as to what your hopes are surrounding the birth, and what they expect is likely to happen on the day.