If spouses' 'hearts be not united in love', their seed could not 'unite to cause Conception', the seventeenthcentury astrologer-physician, Nicholas Culpeper noted. The authors of early modern medical and conduct texts argued that marital compatibility and harmony were necessary for a union to be fruitful. But where historians of sexuality have assumed that such exhortations spoke to the centrality of sexual pleasure, male and female, to conception, this article contends that having a happy and procreative marriage required far more than achieving a certain measure of enjoyment in sex. Working out whether a prospective spouse would be suitable was a complex process that took into account social, financial, emotional, bodily, religious and astrological similarities. Drawing on conduct manuals, childbearing guides, medical casebooks and the accounts of two unhappy wives, Anne Dormer and Sarah Cowper, this article shows that while the frameworks of compatibility and incompatibility in medical and conduct literature seemed to offer a way for talking about unavoidable and conscionable disagreements and childlessness, there was considerable pressure on women, rather than men, to overcome unhappiness and ensure fruitfulness. The seventeenth-century astrologer-physician Nicholas Culpeper explained that there were multiple causes of childlessness. There might be male or female 'impotency in conception'; 1 the womb might be too narrow, so it was not 'apt' to 'receive the Yard [penis] fitly'. Both husbands and wives had to 'spend' their seed, although precisely what this meant was sometimes ambiguous. Even if this seed was jointly emitted, it might be 'unfruitful' because one or both parties were unhealthy, or because there needed to be a 'certain proportion' between their seed and between their constitutions. The 'occult qualities' in seed could either 'agree or disagree'. 2 The 1656 edition of Culpeper's Directory explained that their complexions might be too similar to be able to conceive a child, for it was the 'universal course of Nature' that contraries were like to increase. 3 A third problem could be caused by the womb not 'sucking' the seed or not receiving it in a 'right manner'. Culpeper termed this the 'attractate faculty', which, when impeded as a result of 'distempers of the Womb' or because a 'woman hates her husband', prevented male seed from reaching where it needed to be. Fourthly he noted that sometimes the womb was too 'weak' to retain the seed, even if it could or wanted to attract it. But this too had a close relationship to the tenor of the marriage.