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1-4 weeks pregnant

pregnancy weeks 1-4 Life Begins


Although you may not even know you’re pregnant, by week 4 your body is already going through some significant changes. Learn when your baby’s genetic make-up becomes set and how your placenta will soon begin to form to support them, along with nutrition tips for weeks 1–4 of pregnancy.

The start of your baby’s journey

Fertilisation takes place around 14 days after the start of your last period1. At that point, the fertilised egg that will eventually become your baby is technically called a zygote2.

During the 90 hours after fertilisation, the cells divide and subdivide into a pinhead-sized solid mass of cells3 that gradually makes the journey from your fallopian tube to your uterus2.

"Your baby’s development at 3 weeks"

Around 6-10 days after ovulation you may notice some light spotting as the bundle of cells, now known as a blastocyst, attaches to the wall of your womb4.

“Because of the way pregnancy is measured, you are considered to be 2 weeks pregnant at the time of fertilisation.”

Whether you’re aware of any subtle changes or not, your baby, now known as an embryo, already has the basic building blocks of their body in place. The rapidly multiplying cells form distinct layers within the embryo. These will soon become your baby’s internal organs; the skeleton and muscles; brain; nervous system; and external body parts, such as skin, eye lenses and nails1.

Even though there are incredible developments taking place, at 4 weeks, or 2 weeks after fertilisation, the only sign of pregnancy may be the lack of your usual premenstrual symptoms.

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The placenta: Building a super organ

In the 4th week of pregnancy, your baby is receiving all the nourishment they need from a tiny yolk sac. But very soon your placenta will begin to form1. This life-sustaining organ will supply your baby with the oxygen and nutrients they need to grow and develop. It also passes antibodies to your baby, which will provide essential resistance to infection throughout pregnancy5.

Another function of the placenta is hormone production. These hormones help your baby grow and develop while supporting your own physical changes from the earliest stages of pregnancy. Progesterone and relaxin, for example, both have a relaxant effect on your muscles, allowing your uterus to adapt and make room for your growing baby6.

Vitamin C: Supporting healthy cells for life

Vitamin C is a vital nutrient to include in your balanced diet in the first 4 weeks of pregnancy. As your body adjusts and prepares to support your baby, it plays an important role in keeping cells healthy and contributes to normal collagen function. Collagen gives support and structure to tissue and organs such as skin, blood vessels and cartilage7, and is one of the building blocks of your rapidly growing placenta.

“A balanced diet containing fruit and vegetables can provide all the vitamin C you need in pregnancy.”

The daily recommended nutritional intake (RNI) of vitamin C during pregnancy is 40mg a day, and an extra 10mg a day during the third trimester8. A large strawberry provides around 10mg.

An equally important role of vitamin C is to aid the absorption of non-haem iron (the iron found in plant sources such as spinach) into your bloodstream8. Iron is a key component of your blood, helping to transport oxygen around your body8 and to your baby. A healthy supply of this mineral is essential for your baby’s normal cognitive function9.

Next Steps

Add one of the following vitamin C-rich foods to your mealtimes to increase your absorption of iron from plant sources such as green, leafy vegetables:

  • Steamed broccoli
  • Raw spinach
  • Tomatoes
  • Strawberries
  • Brussels sprouts
  • A kiwi fruit
  • Blueberries
  • Freshly squeezed fruit juice
  • A squeeze of lemon or lime

View references

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1. NHS UK. Pregnancy and baby [Online]. 2013. Available at: [Accessed June 2014]

2. NHS UK. Fertility: the facts [Online]. 2012. Available at: [Accessed July 2014]

3. Deans A. Your New Pregnancy Bible, The experts’ guide to pregnancy and early parenthood. 4th ed. London: Carroll & Brown Publishers Limited, 2013. p. 14.

4. NHS UK. Pregnancy and baby [Online]. 2013. Available at: [Accessed June 2014]

5. NHS UK. What is the placenta? [Online]. 2013. Available at: [Accessed June 2014]

6. NCT. Hormones in pregnancy [Online]. Available at: [Accessed July 2014]

7. European Union. Commission Regulation (EU) No 432/2012 of 16 May 2012 establishing a list of permitted health claims made on foods, other than those referring to the reduction of disease risk and to children’s development and health. OJ L 136 2012;1-40.

8. Department of Health. Report on Health and Social Subjects 41. Dietary Reference Values for Food Energy and Nutrients for the United Kingdom. London: TSO, 1991.

9. European Union. Commission Regulation (EU) No 957/2010 of 22 October 2010 on the authorisation and refusal of authorisation of certain health claims made on foods and referring to the reduction of disease risk and to children’s development and health. OJ L 279 2010;13-7.

Last reviewed: 7th July 2016

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