The first trimester (weeks 1–13)
This is the most sensitive and most rapid period of development, in which the outline of all the basic organ systems is formed.
The heart
Movement of the heart can generally be seen on ultrasound from weeks 5–6. The heart rate rises until week 10 (reaching an average of 171 beats a minute), then falls to around 156 by week 14. In other words, the rate falling after a while is to be expected.
Head and brain
Hardening of the bones of the skull begins at week 11. The hemispheres of the brain grow rapidly; at weeks 11–14 a typical view showing the symmetry of the brain is obtained on ultrasound.
The digestive system — a detail worth knowing
Part of the bowel protruding outside the abdomen in the baby is normal up to week 11 and is a stage of development that resolves on its own. The bowel returns to the abdominal cavity from week 12.
For this reason, this finding seen at an early stage should not be confused with a true opening in the abdominal wall — this distinction is made at the 11–14 week ultrasound.
The stomach and the bladder can be clearly distinguished from week 12.
Arms and legs
Between weeks 11 and 14 all three segments of the arms and legs become visible separately; at this stage the baby can open and close its hands.
Growth
In this period the baby's size is measured by the crown–rump length. In just the three weeks from week 11 to week 14 the baby almost doubles in size — from 45 mm to 84 mm.
The second trimester (weeks 14–27)
This is the period in which the outlines of the organs begin to function and the anatomy becomes clear.
The period of the detailed examination
In the detailed ultrasound carried out between weeks 18 and 22, the four chambers of the heart and the outflow of the main vessels, the kidneys, the integrity of the spine and the profile of the face are assessed. The critical structures in the midline of the brain become clear at weeks 17–20. For details you can see the Screening Tests in Pregnancy page.
When are movements felt?
The baby's movements in fact begin as early as week 12; but the mother does not feel them straight away.
The time they are first felt is generally weeks 18–20. In those who have given birth before this can come as early as week 16; in a first pregnancy it may extend a little beyond week 20. Both are normal.
The frequency and strength of the movements increase until week 32.
How growth is measured changes
From week 14 the crown–rump length is no longer used. Instead, measurements of the head, abdomen and thigh bone are taken and the estimated weight of the baby begins to be calculated.
The third trimester (weeks 28–40)
This is the period in which the preparations for adapting to the outside world are completed and weight is gained most rapidly.
Maturing of the lungs
The substance that allows the baby to breathe on its own and prevents the air sacs in the lung from collapsing begins to be produced rapidly in this period.
If there is a risk of premature birth, drug treatment is given to the mother to speed up the development of the lungs and reduce the risk of breathing difficulty in the newborn — this is what is popularly known as "the injection to develop the lungs".
Movements settle into a pattern
- From week 32 the frequency of movements reaches a level and stays there — it does not decrease.
- At this stage the baby has regular cycles of sleep and wakefulness lasting 20–40 minutes (up to 90 minutes at most).
- Because there is less room, strong kicks are felt instead of large turning movements.
A widespread false belief: "As the baby grows there is no room left, so movements decrease." This is not true. The character of the movements changes but their frequency does not decrease. If you notice a reduction in movements, do not wait — tell your health service; see High-Risk Pregnancy.
Monitoring growth
In this period the baby lays down fat rapidly. Growth is followed at the examination by measuring over the abdomen; if there is a risk it is followed by ultrasound at intervals of 2–3 weeks.
Frequently Asked Questions
When is the heartbeat seen?
Movement of the heart can generally be seen on ultrasound from weeks 5–6. The heart rate rises until week 10 (an average of 171 beats a minute), then falls to around 156 by week 14 — the rate falling after a while is to be expected and is not a cause for concern.
When will I feel the baby's movements?
Movements in fact begin as early as week 12, but the mother does not feel them straight away. The time they are first felt is generally weeks 18–20. In those who have given birth before it can come as early as week 16; in a first pregnancy it may extend a little beyond week 20. Both are normal.
Do movements decrease in the last months?
No — this is a widespread but false belief. From week 32 the frequency of movements reaches a level and stays there; it does not decrease. Because there is less room the character of the movements changes (strong kicks instead of large turns), but their frequency does not fall. If you notice a reduction in movements, do not wait — tell your health service.
Does my baby sleep?
Yes. In the third trimester the baby has regular cycles of sleep and wakefulness lasting 20–40 minutes (up to 90 minutes at most). Short periods without movement may be related to these cycles; but if there is a marked reduction in your usual pattern of movements, it should be assessed.
Is it normal for the bowel to be seen outside at an early stage?
Yes, and it is an important detail: part of the bowel protruding outside the abdomen is normal up to week 11 and is a stage of development that resolves on its own. The bowel returns to the abdominal cavity from week 12. For this reason, at the 11–14 week ultrasound this finding is distinguished from a true opening in the abdominal wall.
What is "the injection to develop the lungs"?
It is a drug treatment given to the mother when there is a risk of premature birth. Its purpose is to speed up the development of the baby's lungs and to reduce the risk of breathing difficulty after birth. The substance that allows the baby to breathe on its own begins to be produced rapidly in the third trimester; this treatment speeds up the process.
How is the baby's size measured?
In the first trimester the crown–rump length is measured — in this period the baby almost doubles in size from week 11 to week 14. From week 14, measurements of the head, abdomen and thigh bone are taken and the estimated weight of the baby is calculated. In the last period growth is followed at the examination by measuring over the abdomen and, if there is a risk, by ultrasound at intervals of 2–3 weeks.
Sources
- ISUOG Practice Guidelines — the 11–14 week ultrasound scan and the detailed second-trimester scan; fetal biometry and growth measurements.
- RCOG Green-top Guideline — the assessment of reduced fetal movements; the course of the frequency of movements through pregnancy.
- RCOG Green-top Guideline — Antenatal corticosteroids to reduce neonatal morbidity and mortality. Speeding up the maturing of the lungs.
- RCOG Green-top Guideline — the monitoring of a baby with growth restriction and the intervals for growth monitoring.
The information on this page is based on the national and international guidelines listed above. The page is reviewed whenever those guidelines are updated.
Practice and Contact Details
The information on this page is general. Please speak to your doctor about your own situation.
- DoctorOp. Dr. Murat Alpay — Specialist in Obstetrics and Gynaecology
- AddressOp. Dr. Murat Alpay Practice, Tekelioğlu Cd. No:51 B Kat:2, Filo Apt., 07160 Muratpaşa / Antalya
- Telephone0505 351 77 88
- HoursMonday – Saturday 09:00 – 18:00 · Closed on Sunday