Weight Gain In Pregnancy

Weight gain in pregnancy is incredibly variable – so how much is too much? Or too little? Sister Lilian weighs in.

The average weight gain in a woman who is not under- or overweight and has an uncomplicated pregnancy, is between 2–4kg in the first 20 weeks, and a further 10–14kg in the second 20 weeks. Generally, one expects an increase of at least 1.5–2kg each month from 20 weeks, but there will be times that this is a little more or less.

What’s the starting point?

Assessing appropriate weight is not without its challenges. A healthy pregnant woman who is not under- or overweight would have a BMI (body mass index) between 18.5 and 24.9, and would have an expected weight gain as above. A useful guide to BMI variations from the norm and suggested weight gain for a singleton pregnancy is as follows:

  • An underweight woman would have a BMI below 18.5 and should gain between 12.7–18kg
  • An overweight woman would have a BMI of 25–29.9 and should gain no more than 6.8–11.36kg
  • An obese woman would have a BMI of 30 or higher and should gain no more than 5–9kg

If she has a twin pregnancy:

  • A healthy pregnant woman who is not under- or overweight should gain between 16.8–24.5kg
  • A pregnant woman who is overweight should gain no more than 14–22.7kg
  • An obese woman should gain no more than 11.4–19kg


The body mass index (BMI) is a statistic first developed by astronomer, mathematician, statistician and sociologist Adolphe Quetelet around 1830–50, although it was only named as such by physiologist Ancel Keys in 1972. It is calculated by dividing body mass in kg by the square of body height (in m). 

The BMI is not infallible and cannot take every determining factor into account, but has proven its worth as a useful guide.

Where has all the weight gone?

Often in the earlier weeks, due to nausea and other digestive discomforts, a woman will not pick up any weight at all – or even lose some. Weight loss in pregnancy may at times be problematic, but not if the mother eats healthily and her baby seems to develop as expected. Some women pick up less weight generally, some more than the average values.

Here’s how an average of 13.6kg is distributed in a woman’s body at the end of pregnancy:

  • Baby 3.4kg
  • Placenta 0.7kg
  • Increased tissue fluid volume 1.8kg
  • Increased uterine weight 0.9kg
  • Increased breast tissue weight 0.9kg
  • Amniotic fluid 0.9kg
  • Increased blood volume 1.8kg
  • Maternal nutrient stores 3.2kg

Caution – baby in progress

  • If a woman is overweight to begin with, it is possible to lose weight while pregnant if eating and exercising sensibly. She may follow sensible and well-known pregnancy eating plans approved by a pregnancy diet specialist – note that this is not always the doctor or midwife! 
  • If a pregnant woman retains a lot of fluid, it can significantly affect her weight and should always be investigated for possible pre-eclampsia or HELLP.
  • If weight gain is excessively low, this needs to be evaluated in terms of overall development of the baby – if there are no developmental delays and the mother also seems healthy, there is usually no reason for concern.

Most importantly, pregnant women should be advised to eat healthy foods 90% of the time and exercise regularly. In most cases, simply being mindful about healthy choices is the key to gaining the appropriate amount of weight during pregnancy.


This article by Midwife Louette Maccallum originally appeared in Sensitive Midwifery Magazine in July 2017.