Sarah Mackenzie, 27, who has a five-year-old son Connor, said initially she didn't understand why she couldn't fall pregnant for the second time.
"When it hadn’t happened after a few months, I went to my GP to find out why," Mackenzie from Noctorum told the Liverpool Echo.
"He politely told me I was overweight and it might help to lose it."
In one year, Mackenzie lost 3st 9lbs and found out she was pregnant on Boxing Day 2015.
Mackenzie, who is now 20 weeks pregnant, said she immediately joined Slimming World after the suggestion from her GP.
At the time, she weighed 13st 7lbs, which she was told was "too much" for her height of 5ft 3in.
The mother, who was studying Law at university, said she was "shocked" she weighed so much, having always been around 10 stone.
"Because Connor is autistic I couldn't study until he had gone to bed, which was often not until 11.30pm or later, because he switched my laptop off and took my text books," Mackenzie said.
"Sugary drinks and food, and energy drinks, were the only things keeping me awake to do what I had to do."
Mackenzie said she would sometimes skip breakfast and lunch, and have an unhealthy evening meal.
After the warning from her doctor, Mackenzie changed her diet and made sure she ate three healthy meals a day.
For breakfast, she had fruit with yoghurt. At lunch she always opted for brown bread for her sandwiches and for dinner she had chicken with vegetables or fish and rice.
Mackenzie also started exercising by running, and completed the 5k Race for Life, 3k Colour Run and 10k Commando Run throughout the year.
She also began swimming, which she enjoyed with her son.
In 12 months, the mother dropped 3st 9lbs and she credits her weight loss for allowing her to fall pregnant.
"I am positive it was losing weight that helped me to get pregnant again," she said, according to The Mirror.
"Once I’ve had my baby and got my family, I am going to stay healthy for them."
According to the NHS, being underweight or overweight can lower your chances of conceiving.
Women who are underweight or overweight ovulate (release an egg) less regularly, or sometimes not at all, compared to women of a healthy weight.
Women with a body mass index (BMI) above 30 are likely to take longer to conceive, and GPs may recommend that they lose weight if they are planning to have a baby.
Wicks, who is most known for his #LeanIn15 video meals on Instagram, said it was his 90-day, Shape, Shift and Sustain (SSS) programme focusing on fat loss that helped the women conceive.
"A woman came up to me and said she'd been trying for a baby for months and months and months," Wicks told FEMAIL.
"She did the plan and basically fell pregnant. She came over crying because she was so emotional and happy."