Newborn calf

Calf Development and Foetal Growth

A heavily pregnant cow must eat enough to ensure healthy foetal growth.
A heavily pregnant cow must eat enough to ensure healthy foetal growth.

Rearing calves from newborns to replacement heifers or for sale can be a challenging and rewarding experience. Producers around the world do their best to maximise growth and health whilst ensuring proper development in their calves. But that is only half of the story.

By the time a calf takes its first breath, it has already been through stages of growth and development which will greatly influence its future. Foetal programming is a relatively new concept. It states that maternal influences during critical periods of foetal growth can have long-lasting influences on the calf, even when it is fully grown.1 To understand how this can take place, it is crucial to understand how the calf develops.


Pregnancy begins when a cow is fertilised, either by a bull or by artificial insemination, and ends with birth. The average cow will gestate her calf for 283 days or around 9 and a half months. However, that can vary from 279 to 287 days depending on breed. Like humans, this time is broken up into three trimesters of around 3 months each.

First Trimester

After fertilisation, the embryo migrates into the uterus and then embeds itself in the uterine lining. Maternal recognition occurs around day 15-192. Chemical signals from the embryo ensure the corpus luteum (CL), or yellow body continues to produce progesterone, maintaining the pregnancy. If this fails to occur, the CL will regress causing a new follicle will start to mature. The cow loses the pregnancy and then returns to cycling. In such situations, it is difficult to tell that the cow was pregnant. Repeat breeders can often fall into this category.

The placenta forms, providing a direct connection between the dam and the developing embryo. This supplies oxygen and nutrients through blood flow. By day 42, the placenta is complete and the embryo is then referred to as a foetus. If the cow is stressed, for example by heat stress or transportation, the placenta may be negatively affected3 and consequently, foetal growth reduced.

By the end of the first trimester, the heart beat is detectable, the sexual organs are developing, and the foetus has the beginnings of many of its vital organs.

Second Trimester

Newborn calf
A healthy newborn calf is the ultimate result of foetal development.

Muscle development begins in the second trimester and the internal organs of the calf grow and develop. The majority of growth occurs during the third trimester. However in the 2nd trimester, the foetus increases in size from about 250g (rat sized) to around 10kg (medium dog sized). As with other stages of pregnancy, the condition of the dam heavily influences the growth of the foetus. If, for example, she has reduced dry matter intake, or goes through a period of negative energy balance, the calf can experience significant growth consequences.

Third Trimester

The third trimester is when rapid growth starts and the lungs develop. During this period, the cow is channelling about 30% of her energy into foetal growth, so adequate nutrition is vital. Many producers use body condition score (BCS) as a simple method of determining cow condition. Producers must take care to ensure cows have access to sufficient feed in these critical periods.

After about 283 days, the cow will go into labour and hopefully produce a healthy calf. Calf birth weight is on average between 41 and 44kg4, although that can depend heavily on breed and other factors. Within two hours the calf should be standing and suckling on the vital colostrum (first milk) of its dam, ready for its next stage in life.


  1. Funston, R. N., Summers, A. F. & Roberts, A. J. Alpharma Beef Cattle Nutrition Symposium: implications of nutritional management for beef cow-calf systems. J. Anim. Sci. 90, 2301–2307 (2012).
  2. Bazer, F. W. Pregnancy recognition signaling mechanisms in ruminants and pigs. J. Anim. Sci. Biotechnol. 4, 23 (2013).
  3. Limesand, S., Camacho, L., Kelly, A. & Antolic, A. Impact of thermal stress on placental function and fetal physiology. Anim. Reprod. 15, 886–898 (2018).
  4. Linden, T. C., Bicalho, R. C. & Nydam, D. V. Calf birth weight and its association with calf and cow survivability, disease incidence, reproductive performance, and milk production. J. Dairy Sci. 92, 2580–2588 (2009).