The Three Trimesters: What Happens in Each Phase of Pregnancy
About 40 weeks lie between the first day of the last period and birth. This time is divided into three roughly equal periods — the trimesters. Each has its own rhythm: in the first, a few cells grow into a complete tiny human; in the second, that human grows and matures; in the third, the body prepares for birth. Knowing the division helps you make sense of every phase.
The <a href="/en/due-date-calculator">due date calculator</a> shows you which trimester you are currently in at any time — along with your current pregnancy week and the progress towards your estimated due date.
First trimester: weeks 1 to 12
The first trimester runs until the completed 12th week (up to 12+6) and is the phase of organ formation. Within a few weeks, the fertilised egg develops a heart, brain, spine and the beginnings of every organ. Around the 6th week the tiny heart starts to beat, and by the end of the trimester the arms, legs, fingers and facial features are in place.
For many pregnant women this is the physically most demanding stage: tiredness, nausea and tenderness in the breasts are typical, all a result of the strong hormonal changes. Because the risk of miscarriage is also highest in this sensitive phase, many couples wait until the end of the first trimester before sharing the news. This period includes the first major check-up with an ultrasound, at which the due date is dated by the doctor.
Second trimester: weeks 13 to 27
The second trimester covers weeks 13 to 27 (13+0 to 27+6) and is considered by many to be the most comfortable time of pregnancy. The nausea usually subsides, energy returns and the hormonal situation stabilises. The baby now grows strongly, develops its senses and begins to hear.
A special milestone is the first noticeable movements of the baby, which many feel between the 18th and 22nd week — often a little later in a first pregnancy. This phase also includes the second major ultrasound (the so-called anomaly scan), which checks the development of the organs. At this point your doctor can usually also determine the baby's sex.
Third trimester: week 28 to birth
The third trimester begins with the 28th week (from 28+0) and ends with birth. Now it is mainly about growing and maturing: the baby gains weight, the lungs mature and the organs prepare for life outside the womb. Towards the end, the baby usually turns into the birth position, head down.
For the mother, everyday life becomes harder with the growing belly — back pain, heartburn and shortness of breath are common. The check-ups now move closer together, and the third major ultrasound is due. From the completed 37th week the baby is considered full term; from that point birth can begin at any time.
A roadmap, not a stopwatch
The division into trimesters is a helpful roadmap, but not a rigid stopwatch. Every pregnancy is individual, and the time windows given are averages. All the figures in this guide and in the <a href="/en/due-date-calculator">due date calculator</a> are a non-binding orientation and do not replace a medical diagnosis.
Follow your journey through the three phases in the <a href="/en/due-date-calculator">due date calculator</a> and discuss any questions about your pregnancy with your gynaecologist or midwife — they will accompany you through every trimester.