The first trimester of pregnancy covers between weeks 1-13, or the first three calendar months. For at least half of this time, you may not even be aware that you are pregnant. The first week of pregnancy is included in the first trimester, in the time when you may be having your period, confusing as it sounds. This is because it is seen as a time marker for when ovulation is most likely to have occurred. For the majority of women this is somewhere around 14 days from the first day of her last period, though the time varies depending on the length of each woman's individual cycle.
Some pregnancy experts talk of gestational age and how this compares with foetal age. Gestational age is the age of the pregnancy and weeks since the last period, while foetal age refers to the actual age of the maturing baby. Throughout this series and commencing with the first trimester, we will talk about gestational age because most of the information around pregnancy does this as well.
How you may feel
By the end of the first trimester most women have had their pregnancy confirmed. If you haven t made an appointment to see a midwife or doctor, make sure you tick it off on your to do checklist. Some of the earliest signs of pregnancy have already been discussed (Check week 6) but here are some others:
- Breast changes including tenderness, tingling, increased sensitivity and a darkening of the areola (the skin around your nipple).
- Needing to wee more frequently than normal. This is because of the increased blood volume as well as your uterus placing pressure on your bladder.
- Feeling tired, not having any energy and wanting to sleep a lot. You may feel dizzy or light-headed because of the changes in your blood sugar levels.
- Feeling nauseated, having morning sickness which might not just be limited to the early part of the day.
- Being moody and easily irritated. You may be a bit tearier and not as even tempered as you usually are.
- It is very normal to feel anxious about the possibility of miscarriage. This most commonly occurs in the early weeks which is why, for most women by the time they progress to the end of their first trimester; they feel they can relax a bit.
How you may look
- At twelve weeks your uterus will be starting to lift out of your pelvis and a small bump may be visible in your lower tummy. This won t be obvious to others, though you may start to show earlier if you have had a baby before.
- Your breasts may increase a lot in the first trimester and look bigger than they normally do. Be prepared to buy new bras in a larger size.
Now we will start to look at each of the 13 weeks in the first trimester in more detail and what you can expect.
Conception usually happens in the fallopian tube and although it is, by any standards a significant moment, there will be no outward signs that fertilisation has happened. Generally, it takes around 6-12 days for the fertilised egg to travel down the fallopian tube to the uterus.
Once the fertilised egg has nestled into the wall of the uterus, signals will be sent to your body to produce more oestrogen and progesterone. These and other hormones will help sustain the baby throughout your pregnancy. Some women will have a small bleed as the embryo implants into the uterine lining.
The placenta has started to form by now and will play a vital role in producing specific hormones and feeding your baby. By this stage, the embryo is smaller than a grain of rice but each of its cells are already programmed to fulfil a specific function.
It is now that a heart beat may be seen on an ultrasound and the eyes and ears of the embryo are being formed. Small buds of tissue start appearing on each side and these will eventually become the limbs.
If you haven t already done a pregnancy test, do one now. By the time you are six weeks pregnant, the baby can officially be measured. It is standard practice when measuring a baby through ultrasound, to measure from their crown to their rump. By week 6 an average size is 5-6 mm.
You are now officially just over half way through your first trimester. Your baby has grown to about 10,000 times bigger than it was when you first conceived. So much of your baby's growth is concentrated on their brain this week that around 100 new brain cells are forming every minute.
This week your baby is around 1 centimetre long and can officially be called a foetus. The valves in your baby's heart are present and the passages which will help air flow from their throat to their lungs have formed. Its fingers, toes and lips, eyelids and legs are becoming more clearly defined.
At this time the mouth and tongue are starting to be formed. The hands which until now have been webbed, will start to separate into fingers. On an ultrasound, the foetus can be seen making jerking movements.
The foetal heat is dividing into four chambers and it can be heard with an ultrasound Doppler. All of the baby's body organs have developed. Their brain is still large and the digestive system is developing.
By now, the foetus genitals have clearly formed, but it's still too early to tell whether you will be having a boy or a girl. Tooth buds are also forming and the eyes are fully developed. Fine hair covers the body and its facial features are more clearly like those of a baby.
By now, the foetus is fully formed and all of its vital organs and nervous system pathways have formed. Until now, the bones have been soft but from now on they start to harden. The foetus is less curled up than it was and starts to straighten.
Your Baby is as cute as a peach this week and around the same size too. Up until this week, their abdominal organs have been forming outside their skin but now they will start moving back to where they should be.
For more information see Pregnancy or Week by Week.
Last Published* December, 2022
*Please note that the published date may not be the same as the date that the content was created and that information above may have changed since.