Early in 1851, Marx wrote to Engels one of his many letters, this one concerning ground rent. Early in the letter, he informs Engels:
An inverse relationship of the fertility of the soil to human fertility must needs deeply affect a strong-loined paterfamilias like myself, the more so since mon mariage est plus productif que mon industries (MECW 38: 274).
Jenny was pregnant with their fifth child. To add a slight complication, Lenchen (Helene Demuth), the real head of the household, was also pregnant, perhaps by the same paterfamilias.