13 Weeks Pregnant

What’s happening this week

Your baby..

  • measures about 7cm
  • weighs about 23 grams
  • all 20 teeth are formed
  • meconium (the baby’s first poo) is developing.


  • are in the second trimester now!
  • are starting to feel pregnant
  • may want to spread the good news if you haven’t already.

The hormone changes of the early weeks begin to settle and your chances of miscarriage are now much lower. This week we explain what the usual blood tests are for.

CVS (chorionic villus sampling)

What is CVS?

It’s a diagnostic test for Down syndrome best carried out at 11 – 13 weeks.

How is it done?

It involves taking a tiny piece of the chorion (part of the placenta) to check the chromosomes of the baby.  The tissue will be tested and you’ll get the results in 7 – 10 days.

What will it tell me?

As it examines your baby’s chromosomes, this test can tell you for certain if your baby has Down syndrome.

Are there any risks?

There is a small risk of miscarriage: around 0.5 – 2% (although it is hard to be accurate because many babies miscarry spontaneously early in pregnancy anyway).

Rhesus status

During your pregnancy, you will have a number of routine blood tests, with your first blood test they will find out your blood group (A, B, O or AB) and your rhesus status (positive or negative).  People who are rhesus (Rh) positive have a protein attached to their red blood cells, those that don’t have the protein are known as Rh-negative.  Most women in NZ are Rh-positive.  Your Rh status is only significant if an Rh-negative mum is carrying an Rh-positive baby (the baby will have inherited this from an Rh-positive Dad).  If some of your baby’s blood gets into your bloodstream, your immune system may react to it as if it were a ‘foreign invader’ and produce antibodies against it.  This is known as sensitising.

Sensitising is not usually harmful in the first pregnancy but can be in subsequent pregnancies because when we make antibodies they stay in the blood forever.  If a subsequent baby is Rh-positive the antibodies can cross the placenta and invade the blood cells of the developing baby.  This could cause anaemia or jaundice (and rarely more serious problems) in the baby.

Fortunately, a substance called Anti-D is available for Rh negative women which prevent the formation of those antibodies.  This is given as an injection at any time a sensitising event may have occurred for example – miscarriage, a bleed in pregnancy, amniocentesis/CVS, car accident, fall or at the birth.

Not enough iron?

Many women need extra iron when they’re pregnant. A lack of iron can make you feel tired and breathless. If your iron levels are low, your LMC will prescribe iron tablets, but there are simple ways to help keep your levels up. Eat meat or fish with other iron-rich foods.

  • Eat meat or fish with other iron-rich foods.  Baked potatoes with beef chilli or a hot chicken sandwich make good meals.
  • Drink a glass of orange juice with your meal; vitamin C helps you absorb iron.
  • Drink less tea; the tannin in tea reduces the amount of iron you can take in.
  • Treat yourself to the occasional bar of chocolate!  Dark chocolate especially is high in iron.
  • Add dried fruit like raisins and apricots to your bowl of breakfast cereals.

For more helpful tips of what to eat when pregnant, check out our What to Eat section.