Heidi Having Baby Fever
After Spencer and Heidi got married, Heidi started to feel like having a baby. Spencer on the other hand, did not want one yet. Throughout the show we saw Heidi do everything in her power to trick Spencer into having kids with her. Thankfully, the Speidi baby plot line was fake, according to Heidi.
Source: http://www.mtv.com/news/1619750/heidi-and-spencer-to-contemplate-kids-on-the-hills/