Twin births

A few of us were talking to Pat the Salt last weekend about growing up in Cardiff before WWII. He ran away to sea at the age of 14 in 1939 which was perhaps an unfortunate time to join the merchant marine. It's different for teenage boys of course because they are immortal super-heroes and getting torpedoed in the middle of the night was rather an adventure. aNNyway, Pat was born at home in Longcross St, Adamsdown, Cardiff and he volunteered the information that he weighed 11 lb = 5kg at birth. That is a truly frightening lump for a vaginal midwife-assisted delivery but he's been telling the story for decades and I have no reason to doubt that is what came down through family lore. The other folks in the room turned to/on me with  "You're a twin, how much did you weigh?"

Well our family lore is that my poor mother looked and felt like a beached whale and was taken for a long and bumpy drive by the midwife across the fields on the top of the white cliffs of Dover [cue Vera Lynn]. That didn't help much and my sister and I went to full term. I was 7.5 lb / 3400g and The Sister was 5.5 lb / 2500g. Family lore goes on to report that the smaller sib was born without hair or fingernails with the implication that I had committed placental robbery. It didn't do me much good because, for all the years that mattered when we were growing up, my sister was smarter, kinder and calmer than me.  The question hanging in Pat's kitchen was whether in sex-discordant twins the boy child is always larger and it was up to me to find out if their was any data on the subject.

Well it's really difficulty to find an scientific study that says that androgens act to preferentially corner the available nutrients in the male twin. So I conclude that there is no such effect. A 1990 study found such an effect, but the sample size was really small, the significance of the trend was marginal [p = 0.04] and only manifest in the cases where the male child was delivered first (like me). Another very small sample suggests that discordant M/F twins seem to pack on the weight more than concordant twins but note that the error bars on these numbers is about half a kilo so that swamps out the idea that these trends are significant:

Birthweight	MF	MM	FF
F	2100	-	1800
M	2150	2000	-

This makes me feel double guilty because I cannot, like Buster Gonad, blame my unfeasibly large testicles on the weight differential back in one hospital in Dover in 1954. There is a sense nevertheless that androgens produced by the male twin do leak across the placenta to affect the growth and development of the female twin.

It's different in cattle because the placenta in Bos taurus has a different structure to that found in primates like us. In cattle, the chorions of twins usually merge and there is a much greater exchange of fluids between the two fetuses. The effect of male hormones on the female calf is much more dramatic and such heifers are called freemartins and are usually effectively infertile. The bull-calf tends to have small testicles and this may also have an effect on fertility. The rate of twinning [1/200 pregnancies] in cattle is much less than in sheep, for example and only half of those twin-pregs will be M/F. Although all sorts of wild and wonderful births have been observed in humans, freemartins are extremely rare because the chorions of the human placenta are generally not shared. The exception being in some monozygotic (identical) twins who will be the same sex anyway.

One of the minor burdens of having a twin sister is being asked if we are identical.  Usually it is enough to adopt a quizzical / ironic expression and point with both index fingers at my groin.