The Dallas Mavericks basketball star and his wife Khloe, Kourtney's sister, have been desperate for a child of their own since tying the knot in 2009 and Lamar confessed seeing his sister-in-law pregnant has made him realise what he is missing out on.
The sportsman - who has two children, Destiny, 13, and 10-year-old Lamar Jr., from a previous relationship - revealed his heartache in a preview clip of the second series of his reality TV show 'Khloe and Lamar'.
When asked by Khloe "Were you jealous that Kourtney got pregnant again?" he answered "Yes, they get to have another baby and I don't."
Khloe is reportedly seeing fertility experts in a bid to get pregnant and has been researching top specialists in Dallas, Texas, following her and Lamar's move there late last year from Los Angeles.
A source close to Khloe - who also underwent treatment in her home town of Los Angeles - said: "Khloe has been meeting with fertility doctors in Dallas, and Lamar has been going with her to several appointments. It makes sense for her to have a fertility doctor in Dallas, as well as Los Angeles. Khloe very much wants to have a baby with Lamar, and their inability to get pregnant has had its rough moments, but this has only made their relationship stronger."