Pregnancy Week 13 – The Baby’s Progress

By pregnancy week 13, the placenta, which has recently taken over from the yolk sac as your baby’s vital life support system, until the day he is born, will continue to gradually increase in size now throughout the rest of your pregnancy. The placental layers will thicken and grow until the membrane covers the entire inner surface area of the uterus. Unlike the yolk sac, the placenta will not be responsible for the production of blood cells, this will now be undertaken by your baby’s own bone marrow, liver and spleen which are now fully developed.

Pregnancy Week 13

The importance of the placenta is to act as a filter between mother and baby, carrying oxygen rich blood and nutrients from the mother through to the fetus and returning carbon dioxide and waste products back through to the mother.
This transfer of essential life support goes back and forth via the umbilical cord. This is why it is essential to always maintain a healthy diet throughout your pregnancy, without toxin intakes such as alcohol, as this could cross the ‘placental barrier’ and have adverse effects on the unborn.

Simply put, as de-oxygenated blood is returned from the baby to the mother, the arteries in the umbilical cord form a complex network of capillaries at the junction of the placenta in order to bypass any chance of the maternal and fetal blood becoming contaminated with one another. To ensure further protection of the fetus, the placenta will also release the hormone Neurokinin B which will prevent the mother’s immune system from attacking the ‘rapid cell production’ otherwise known as your baby.

The placenta is a dark red/maroon colored organ in the shape of a disc that is thick in the middle and thin around the edges. Now you’ve arrived at the 13 week pregnant phase the placenta will not have reached it’s full size yet but when it does it can be around 21-23cm (roughly 9”) edge to edge and around 2.5cm (1”) thick in the middle and weigh around 500g (1.25 lbs).

There are a number of pathologies can effect the placenta resulting in pregnancy complications.

These are:

  • Placenta Accreta
  • Placenta Praevia
  • Placental Abruption

Diagram shows structure of the fetal placenta formed in pregnancy

Along with the growing placenta, your 13 week fetus is also showing a lengthening of the limbs, with the legs becoming a little longer than the arms, although it’s the arms that he will learn to use first. The distinct difference in the appearance between his hands and feet is also now noticeable.

The genitalia is also taking a more prominent shape with external swellings that will soon be visibly identifiable as either male or female.

Pregnancy week 13 will see your baby growing to around 3.07″ (78mm) crown to rump and weighing around 1.23oz (35 g).

<< Week 12

Week 14 >>

(See 13 Weeks Pregnant – The Mother’s Progress)


Return from PREGNANCY WEEK 13 to HOME