At this stage, there’s not much to see. Your newly-fertilised egg is called a zygote¹. 90 hours after fertilisation, cells divide and subdivide into a tiny mass that will travel from your fallopian tubes to your uterus. After four weeks, your baby will be the size of a poppy seed and will be protected by an amniotic sac. This is filled with cushioning fluid, and attached to a tiny yolk sac that provides your baby with nourishment. The outer layer will develop into the placenta, providing your baby with oxygen and nutrients².

It’s unlikely you’ll look pregnant: most first-time mums don’t start showing until at least week 12. It’s unlikely you’ll feel pregnant, too, but that will change when the ‘pregnancy hormone’ - human chorionic gonadotrophin² - becomes active. That said, you’re starting to grow a brand new organ - the placenta.

By week 4 of pregnancy, your baby will get their nourishment from a tiny yolk sac, but this will be replaced by the placenta². This brand new organ will supply your baby with oxygen and nutrients needed to grow and develop. It also passes antibodies to your baby, for resistance to infection throughout pregnancy³.

Another function of the placenta is hormone production³. These hormones help your baby grow and develop while supporting your own physical changes. Progesterone and relaxin, for example, both have a relaxant effect on your muscles, allowing your uterus to adapt and make room for your growing baby⁴.

Early pregnancy symptoms⁵ can be subtle, and are sometimes missed. That said, it’s possible you may experience some of the following:

Last reviewed: 13th May 2022
Reviewed by Nutricia’s Medical and Scientific Affairs Team