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What do Weeks and Trimesters Mean in Pregnancy?

It can become a little confusing determining the number of weeks you are Pregnant and the Trimester that you are in.   To help clear this up there are 2 different ages that can be considered:

Weeks Pregnant & EDD (Expected Due Date)

When determining the number of Weeks Pregnant you are it is Gestational Age that is referred to

  • Gestational Age – determined with 0 being the 1st day of your last period (menstruation)
  • Fetus Age – determined with 0 being the day of Conception (when the egg fertilized during your fertile period (Ovulation time))

It is Gestational Age that Obstetricians use when they perform Obstetric Ultrasounds and they use this age to determine when various screening tests and assessments of the fetus and mother need to be carried out.  So when your Obstetrician confirms the number of weeks Pregnant you are this is according to Gestational age and calculated from the 1st day of your last period.   Ie.  if the first day of your last period was on 1st March then on 1st April you will be 4 weeks Pregnant.

They will classify your EDD (Expected Due Date) to be 40 Weeks after First Day of your Last Period.


Trimesters are based on the weeks.

1st Trimester – Gestational Age 0-13 Weeks (Fetal Age 0-11 Weeks)

This the time that the Fetus will undergo extensive developments with developing their Nervous system, Brain, Heart and Circulatory System and why staying away from any external factors that could affect their development such as Alcohol, Smoking & Drugs is so important.

2nd Trimester – Gestional Age 14-26 Weeks (Fetal Age – 12-24 Weeks)

This period is usually referred to as the Honeymoon period as usually a lot of the unwanted symptoms of early pregnancy will subside.  Vital organs will continue to develop and the baby will continue to grow in size.   Movements of the baby will start to be felt during this period.

3rd Trimester – Gestational Age 27-40 Weeks (Fetal Age – 25-38 Weeks)

This period is the homestretch of the Pregnancy where the Fetus, Your Uterus and your Tummy will continue to grow.

4th Trimester – the 12 Week Period after Your Baby is Born 

This period is just as important regarding a mothers health as the 1st 3 Trimesters yet often a time that mums and their baby do not receive the support they require.   Here is a list of Health Professionals can help in this time.  You can find a list of Providers in your area through FIND.

  • Postnatal Midwives & Doulas – can usually visit you at hospital, home or online and help answer any questions and support you
  • Lactation Consultants – can help with getting a good Latch and sort any issues you may be facing with Breastfeeding or Pumping
  • Pediatricians – help with any concerns related to your baby
  • Womens Health Physiotherapists – can relieve any pains and check your Pelvic Floor and Ab muscles after birth
  • Chiros and Osteos – can help relieve any pains and help babies with issues such as Torticollis, Misshapen heads, Inturned Feet
  • Psychologists & Counsellors – can help with any Postnatal Blues

Clarifications of Preterm to Post Term Birth

Depending on when your baby is born it determines how the birth is classifed depending on Stage of Pregnancy

Preterm (20-37 weeks)

Babies may be born preterm due to a spontaneous unplanned labour or medically there is an indication to induce labour or have a planned c-section earlier.  Preterm can be split further into:

  • Extremely Preterm – before 28 weeks
  • Very Preterm – 28 to 31 weeks + 6 days
  • Moderate to Late Preterm – 32 weeks to 36 weeks + 6 days

Early Term to Post Term (37 weeks+)

  • Early Term – 37 weeks to 38 weeks + 6 days
  • Full Term – 39 weeks to 40 weeks + 6 days
  • Late Term – 41 weeks to 41 weeks 6 days
  • Post Term – 42 weeks+

1st Trimester

Here is the 1st Trimester broken down into key changes that happen:

Gestational Age – Weeks 14-16 (Fetal Age – Weeks 12-14)

  • Skin of fetus is transparent and lanugo fine hair begins to form on the head
  • Fetus will now begin sucking and swallowing tiny amounts of the amniotic fluid
  • Meconium which is formed within the now functioning intestinal tract will be building up & will lead to babys first bowel movement after birth
  • Feelings of Flutters in the tummy may begin now the fetus is starting to move around more
  • Fingerprints have now developed on their tiny fingers.

Gestational Age – Weeks 17-20 (Fetal Age – Weeks 15-18)

  • Eyebrows and eyelashes will be growing
  • Nails will have begun to grow on their fingers and toes
  • By about the 20th week they will have Vernix which is a white coating the baby will have when first born which protects them from the amniotic fluid

Gestational Age – Weeks 21-23 (Fetal Age – Weeks 19-21)

  • The fine Lanugo hair will now be all over their body and the skin will become less transparent looking
  • Fat beneath the skin will start to develop
  • All the different parts of the eye including the pupil and iris will become fully developed

Gestational Age – Weeks 24-26 (Fetal Age – Weeks 22-24)

  • This stage is an important stage as your fetus if is born early they will have a chance of survival with medical technology assistance
  • Lungs air sacs will have begun to form
  • Your fetus will have started sleeping and waking cycles

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