[–] ProxyMusic 5 points Edited

I got in a full blown raging argument with some idiot on Instagram about this the other day. You can’t see the sexes characteristics for a few weeks but it’s literally determained when your conceived.

I dunno what you mean by "see" and "sex characteristics" there, but cell biologists have observed and documented clear sex differences in the blastocysts of various mammals including humans just days after fertilization prior to when blastocysts reach the stage of development where they implant in the wall of uterus.

Numerous sex differences have also been found in the placental cells of males and females of different mammal species starting from the time the blastocyst implants in the uterus and starts growing a placenta and thus first becomes an embryo.

(During fertilization, the sperm and egg unite in one of the fallopian tubes to form a zygote. Then the zygote travels down the fallopian tube, where it becomes a morula. Once it reaches the uterus, which usually happens 3-5 days after fertilization, the morula becomes a blastocyst. The blastocyst then burrows into the uterine lining — a process called implantation. The implantation typically starts about 6 days after fertilization.)

It's true that by looking at the external morphology of embryos from the outside, "you can't see sex characteristics" such as gonadal differentiation until weeks following fertilization (6 weeks in humans, to be precise). But by zooming in closer and looking at cells with the kinds of microscopes, imaging technology and research methods that scientists in cell biology have been using for years, sex differences are visible within days of fertilization/conception. Sex differences are probably there from the get-go, it's just that scientists haven't documented them yet due to technical limitations - limitations that no doubt will be overcome in due course.

The big takeaway of cell biology research done in the past 30 years, and especially in the era of human stem cell research that began in the late 1990s, is that sex differentiation in mammals including humans occurs and can be seen by scientists long before the gonads differentiate and the genitals begin developing - indeed, sex differences can be seen and have been documented by scientists just days after fertilization.

This and other research done in the past 30 years completely refutes the theory in vogue for most of the 20th century which posits that physical sex differences are mainly the result of differences in male and female sex hormones that occur following gonadal differentiation and development - which also just happens to be the theory that transgenderism is based on. As it turns out, the importance of sex hormones appears to have been way over-estimated because the vast majority of the thousands of physical sex differences between males and females seem to be much more the result of genetics than of sex hormones.

In mammals... male blastocysts develop more quickly than female blastocysts [1, 2], and expression of several genes, such as murine Xist [4, 5], bovine G6PD [6, 7], ZFX [7], HPRT [6] and INF–t [8], and murine Zfy and Sry [9], is different in each sex. However, there is no report on global differences in gene expression in male and female blastocysts, largely because of the technical difficulty in sexing many blastocysts quickly and accurately.

In eutherian mammals, male preimplantation embryos develop more rapidly than females in a number of species, such as the mouse [1, 2], cow [16, 17], human [18], sheep [19], and pig [20]. These differences precede gonadal sex commitment. Here we found minor but statistically significant sex differences in the expressions of nearly 600 genes. These may have arisen from slight differences in developmental stages between the male and female blastocysts, so we cannot conclude that all are associated with sex differentiation. However, we have demonstrated here that at least four genes are certainly expressed differently, not by stage but by sex. We presume that the actual number of genes involved in such sex differences is likely to exceed the small number of previously reported genes [4–9].


The apparent male-specific growth phenotype has been observed as early as the pre-implantation period in several mammalian species, including humans. In vitro pre-implantation studies have identified that XY bovine and human embryos have an overall increase in total cell mass, when compared with XX embryos.

The mechanisms that contribute to these sex-specific differences in growth rates prior to implantation and establishment of the feto-placental unit may be, in part, the result of chromosome X-inactivation [3,22]. This female-specific alteration during early development may have implications for transcript expression and subsequent cellular function.

gene ontology analysis of differentially expressed transcripts identified male trophoblast cells had enriched protein translation and certain mitochondrial and ribosomal functions, whereas female trophoblast cells had an enrichment in transcripts involved in immune function and responses to various compounds and stimuli. Overall, the pre-implantation male embryo and the early gestation male placentae preferentially increase transcripts involved in growth regulatory pathways, whereas age-matched females appear to prioritise alternative pathways. These sex-specific differences likely contribute to an observed increase in male growth rates and outcomes during early gestation in placental mammalian species including humans [1,27], but are also thought to contribute to the observed sex-specific differences in growth trajectories and outcomes during the second and third trimesters [12](Figure 1).


Both in vitro and in vivo bovine female blastocysts are known to produce more interferon tau (maternal recognition signal in ruminants) than male blastocysts (Kimura et al., 2004; Larson et al., 2001), suggesting that female blastocysts have a greater ability to signal their presence to the mother than male embryos. Sexual dimorphism in amino acid utilization has been suggested in both in vitro cultured and in vivo bovine blastocysts (Sturmey et al., 2010). Further, bovine conceptuses exhibit sex-specific differences in amino acid utilization in vivo during the peri-implantation period, with less asparagine, histidine, glutamine, arginine, tryptophan, methionine, phenylalanine, isoleucine, and lysine present in uterine flushings associated with female conceptuses compared to male conceptuses on GD19 (Forde et al., 2016). These findings were accompanied by greater expression of mRNAs encoding amino acid transporters SLC6A19 and SLC1A35 in female conceptuses compared to male conceptuses on GD19. Greater abundances of these amino acids in uterine flushings from male pregnancies may be indicative of less uptake by the male conceptus, which suggests less utilization of these nutrients by male conceptuses during the peri-implantation period of pregnancy.

Staggeringly, transcriptomic analyses revealed sex-specific expression of approximately one-third of all actively expressed genes in the Day 7 bovine blastocyst (Bermejo-Alvarez et al., 2009). This continues throughout the peri-implantation period, with sex-specific expression of >5000 transcripts by Day 19 bovine conceptuses (Forde et al., 2016); this highlights the critical importance of embryonic sex during early conceptus development and